Customize New User Form - in Manager->Security->Manager Users

<?php
	
	$eventName = $modx->event->name;
	
	switch($eventName) {
		
		case'OnUserFormPrerender':
			/* nothing */
			break;
		case'OnUserFormRender':
			$modx->regClientStartupScript('/manager/defaults-for-user.js');
			break;
	}
	
	return;

function addLoadEvent(func) {
	var oldonload = window.onload;
	if (typeof window.onload != 'function')
	{
		window.onload = func;
	}
	else
	{
		window.onload =
			function()
			{
				if (oldonload)
				{
					oldonload();
				}
				func();
			}
	}
}

addLoadEvent(
	function()
	{
		var myForm = document.forms[0]; // getElementById('ext-gen75');   //ext-gen76
		
		
		// set defaults
		var mySpecifyPasswordRadio = document.getElementById('modx-user-password-genmethod-s');
		mySpecifyPasswordRadio.checked = 'checked';
		
		var myCountryField = document.getElementById('modx-user-country');
		myCountryField.value = 'United States';
		var myCountryField = myForm.elements['country'];
		myCountryField.value = 'United States';
		
		var myActiveBox = document.getElementById('modx-user-active');
		myActiveBox.checked = true;
		
		
		// hide other fields
		
	}
);

function changeForm()
{
		var myForm = document.forms[0]; // getElementById('ext-gen75');   //ext-gen76
		
		
		// set defaults
		var mySpecifyPasswordRadio = document.getElementById('modx-user-password-genmethod-s');
		mySpecifyPasswordRadio.checked = 'checked';
		
		var myCountryField = document.getElementById('modx-user-country');
		myCountryField.value = 'United States';
		var myCountryField = myForm.elements['country'];
		myCountryField.value = 'United States';
		
		var myActiveBox = document.getElementById('modx-user-active');
		myActiveBox.checked = true;
		
		
		// hide other fields
		
}


setTimeout("changeForm()",1250);

<?php
	
	$eventName = $modx->event->name;
	
	switch($eventName) {
		
		case'OnUserFormPrerender':
			/* nothing */
			break;
		case'OnUserFormRender':
			if($my_mode == modSystemEvent::MODE_NEW)
			{
				$modx->regClientStartupScript('/manager/defaults-for-user.js');
				$modx->regClientStartupScript('/manager/hide-user-fields.js');
			}
			elseif($my_mode == modSystemEvent::MODE_UPD)
			{
				$modx->regClientStartupScript('/manager/hide-user-fields.js');
			}
	}
	
	return;

function changeForm()
{
	var hide = 'none';
	
	// testing
	//var myTest = document.getElementById('modx-user-phone');
	//myTest.value = 'NEW User';
	
	// grabs the form, there is only one form to begin with
	var myForm = document.forms[0]; // getElementById('ext-gen75');   //ext-gen76
	
	
	// sets the default values on the form
	var mySpecifyPasswordRadio = document.getElementById('modx-user-password-genmethod-s');
	mySpecifyPasswordRadio.checked = 'checked';
		// shows the password fields
		var myPasswordBox = document.getElementById('modx-user-panel-newpassword');
		myPasswordBox.style.display = '';
	
	var myCountryField = document.getElementById('modx-user-country');
	myCountryField.value = 'United States';
		// sets hidden field - THIS IS IMPORTANT!!! - if the field has a hidden field right next to it, then the hidden field needs to be set
		var myCountryFieldHidden = myForm.elements['country'];
		myCountryFieldHidden.value = 'United States';
	
	var myActiveBox = document.getElementById('modx-user-active');
	myActiveBox.checked = true;
		// sets hidden field - THIS IS IMPORTANT!!! - if the field has a hidden field right next to it, then the hidden field needs to be set
		var myActiveFieldHidden = myForm.elements['active'];
		myActiveFieldHidden.value = '1';
	
	
	
	// hide all tabs for new users only, except the first and last one (will attempt to set default Access Permissions later)
	var myListItems = document.getElementById('modx-user-tabs').getElementsByTagName('li');
	for( var x = 1; x < (myListItems.length - 1); x++ )
	{
		myListItems[x].style.display = hide;
	}
	
	
	// hide fields for new users only
	// names of the fields to hide
	var names = new Array();
	names[0] = "modx-user-passwordnotifymethod-e";
	names[1] = "modx-user-password-genmethod-g";
	names[2] = "modx-user-active";
	names[3] = "modx-user-fs-blocked";
	// levels to climb up to hide the entire row that contains the field
	var levels = new Array();
	levels[0] = 8;
	levels[1] = 8;
	levels[2] = 3;
	levels[3] = 0;
	
	// climbs up to the correct level and hides that row
	for( var x = 0; x < names.length; x++ )
	{
		var tempField = document.getElementById( names[x] );
		var i = 0;
		while( i < levels[x] )
		{
			var tempField = tempField.parentNode;
			
			i++;
		}
		
		tempField.style.display = hide;
	}
	
}

// delays call so that the form loads
setTimeout("changeForm()", 1500);

function hideFormFields()
{
	// hide other fields
	var hide = 'none';
	
	// names of the fields to hide
	var names = new Array();
	names[0] = "modx-user-phone";
	names[1] = "modx-user-mobilephone";
	names[2] = "modx-user-address";
	names[3] = "modx-user-city";
	names[4] = "modx-user-fax";
	names[5] = "modx-user-state";
	names[6] = "modx-user-zip";
	names[7] = "modx-user-website";
	names[8] = "modx-user-dob";
	names[9] = "modx-user-gender";
	names[10] = "modx-user-class-key";
	names[11] = "modx-user-comment";
	// levels to climb up to hide the entire row that contains the field
	var levels = new Array();
	levels[0] = 2;
	levels[1] = 2;
	levels[2] = 2;
	levels[3] = 2;
	levels[4] = 2;
	levels[5] = 2;
	levels[6] = 2;
	levels[7] = 2;
	levels[8] = 3;
	levels[9] = 3;
	levels[10] = 2;
	levels[11] = 2;
	
	// climbs up to the correct level and hides that row
	for( var x = 0; x < names.length; x++ )
	{
		var tempField = document.getElementById( names[x] );
		var i = 0;
		while( i < levels[x] )
		{
			var tempField = tempField.parentNode;
			
			i++;
		}
		
		tempField.style.display = hide;
	}
	
}

// delays call so that the form loads
setTimeout("hideFormFields()",1500);