How do I set the url to make an ajax request for a html document?

Yes, there is such way: you should pass absolute path in there…

Let’s say your JS file is enqueued like this:

wp_enqueue_script( 'my-script', 'path-to-js-file.js'... );

Then you should use wp_localize_script and pass the value in there:

wp_localize_script( 'my-script', 'MyScriptData', array( 'ajax_url' => site_url('/test/file.html') ) );

Then you can access this value in your JS file this way:

return $.ajax({
    type: 'get',
    dataType: 'html',
    url: MyScriptData.ajax_url, // <-- here you get that value...
    success: function (resp) {
        alert(‘Success’);
    },
    error: function (jqXHR) {
        alert('Error ‘);
    }
});

BUT…

You should know, that you SHOULD process AJAX requests using proper actions and not with some external file. Here you can read more about dealing with AJAX in WordPress: AJAX in Plugins