There are a billion ways to achieve what you want. I am not even sure where to begin because there are ...
Questions for you...
Can you paste your template, so that we know what's going on?
Is the UL list generated on fly by a return script?
Do you have access to the UL markup?
What have you tried so far and what issues did you get?
Are you using TinyMCE 4+
Here is a working example that will preserve the
itempProp for all subsequent items; on save and, on load.
Learn more from the neatly organised and precise docs:
https://www.tinymce.com/docs/plugins/template/:
templates: [
{
title: 'My auto UL Template 1',
content: "<ul><li {$recipeAttribute}>Item 1</li></ul>"
}
],
template_replace_values : {
recipeAttribute: "itemprop='recipeIngredient'" //careful apostrophe !!
},
extended_valid_elements:"li[itemprop|class|etc|etc]", //add itemprop to list of your allowed attributes or else TinyMCE will strip it as non standard