<form action="<?php echo esc_url( admin_url('admin-post.php') ); ?>" method="post">
<input type="hidden" name="action" value="your_action_name">

add_action( 'admin_post_nopriv_your_action_name', 'your_function_to_process_form' );

function your_function_to_process_form(){
// process your form here
}