I don't understand the question. If you are using the countryoptions snippet, it generates the select for you, and the select is like any hand-coded select except for the bit about maintaining the selected status. The name of the select is the same, so you would just store its value as usual.
Here's how I used it a few weeks ago with a simplecart checkout form. The submitted form will have $_POST['country'] just like any form. The placeholder is named 'fi.country' simply as a convenience for preventing any conflict with any other placeholders.
<div>
<label for="checkout_country">[[%simplecart.orderdetails.address.country]]: <span>*</span></label>
<select class="select required" name="country" id="checkout_country">
[[!FormItCountryOptions? &selected=`[[!+fi.country]]`]]
</select>
<label class="error">[[+fi.error.country]]</label>
</div>
☆ A M B ☆