Best practice validating extension admin settings via ajax

[rpariesnexad95]

Hello,
I have inherited a magento 2 extension. I am a complete noob to magento
In my extension under admin->stores->Configuration->My extension->Settings i need to validate  a username and password via an ajax call to my server.

I have found the fields needed in etc/adminhtml/system.xml
Do i add a backend_model(currently i do not have anything like that) ?::

What type of class is extended for something like this backend_model Class?

<field id="username" translate="label comment" type="text" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
<label>User Name</label>
<comment>Use {{username}} as API Username placeholder</comment>
<backend_model>VendorMe\OnlinePayment\Model\Config\Source\Admin</backend_model>

</field>

Then in that class do i do something like a before_save???
How does this work when you need to validate 2 fields dependent on each other? These appear to be per field.

As you can tell i am completely lost. If anyone can point me to an example that would be the best. But any help would be greatly appreciated 

[faridjamile234]

This short example uses PHP to write our JavaScript in the footer of the page. This script then triggers the AJAX request when the page is fully loaded:

<?php
add_action( 'admin_footer', 'my_action_javascript' ); // Write our JS below here

function my_action_javascript() { ?>
	<script type="text/javascript" >
	jQuery(document).ready(function($) {

		var data = {
			'action': 'my_action',
			'whatever': 1234
		};

		// since 2.8 ajaxurl is always defined in the admin header and points to admin-ajax.php
		jQuery.post(ajaxurl, data, function(response) {
			alert('Got this from the server: ' + response);
		});
	});
	</script> <?php
}

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