You can use get_the_author_meta
function. You can learn more about it here: https://developer.wordpress.org/reference/functions/get_the_author_meta/
Usage in your case:
<a href="https://wordpress.stackexchange.com/questions/290522/mailto:<?php echo get_the_author_meta("user_email');?>?subject=<?php the_title(); ?>">Apply</a>