learndash_register_form

 <?php
/**
 * Example usage for learndash_register_form action.
 */
add_action(
	'learndash_register_form',
	function() {
		$course_id = learndash_get_course_id( get_the_ID() );
		if ( ! empty( $course_id ) ) {
			?><input name="learndash-registration-form-course" value="<?php echo $course_id; ?>" type="hidden" />
			<?php
			wp_nonce_field( 'learndash-registration-form-course-' . $course_id . '-nonce', 'learndash-registration-form-course-nonce' );
		}
	}
);