How to define a filter in plugin before plugin load?

Hi all,

I have the functions code below in plugin:

if( !function_exists('tourmaster_get_enquiry_form') ){
	function tourmaster_get_enquiry_form( $post_id = '' ){

And I want add filter for it like this:

if( is_admin() ){ add_filter('the_enquiry_form', 'tourmaster_get_enquiry_form'); }
	if( !function_exists('tourmaster_get_enquiry_form') ){
		function tourmaster_get_enquiry_form( $post_id = '' ){

But the filter not work :(, I’ve tried ask the dev and he told me I must define the filter before the plugin load to make it work. It’s quite difficult for me because I don’t familiar with PHP and WordPress Codex. I hope I can get some advice to learn about it.

Thank to advance for any help!!!