The form action is the file that handles the form, which you can see is the admin-post.php file in your WordPress installation.
However what you are really looking for is the hook that handles the form.
Basically, the form is getting submitted to admin-post.php, which then uses the “action” value in your form to determine what hook to trigger; in other words, what code to run. This is a common WordPress paradigm.
The hook name is generated in this format:
admin_post_{{action}} for logged in users, and
admin_post_nopriv_{{action}} for non-logged in users
In your case the action is set to “check_imported”. So if you are trying to find code that is already handling these, do a search in your theme/plugins for “admin_post_check_imported” and/or “admin_post_nopriv_check_imported”.
They would be written as an add_action call, connecting the hook name to some other function, like:
add_action( 'admin_post_check_imported', 'some_function_name');
So you would then look for the function named, “some_function_name”, for the actual code.