Filters the start course button on single course and archive course templates.
$button (string) entire “start course” button which links to first lesson in its respective course.
add_filter( 'wpc_start_course_button', 'filter_start_course_button' );
function filter_start_course_button( $button ) {
$id = get_the_ID();
$link = get_the_permalink($id);
return '<a href="' . $link . '">My Custom Button</a>';
}