Yithemes Ajax Search Button to Use GP font icon

Hello,

In the header I am attempting to use GP font for search icon, in a Yithemes ajax product search.
I have read and tried to follow previous forum posts on this topic.

PHP function file:
add_filter( 'generate_search_button','generate_custom_search_button' );
function generate_custom_search_button()
{
return '';
}

CSS:
#yith-searchsubmit { font-family: GeneratePress;}

I can use font-awesome but prefer not to.

Thank you very much for your help!