fluent-forms-learndash-course-enrollment.php

<?php
/*
* This snippet is assuming the user already logged in or the provided email address is available in users
* After the form submission the user will be added to the course. 
*/

add_action( 'fluentform_submission_inserted', 'ff_register_user_to_learndash_course', 100, 3 );

function ff_register_user_to_learndash_course( $entryId, $formData, $form ) {
// Check if the form is the one we want
    if ( $form->id != 123 ) { // Replace with your actual form ID
        return;
    }

    if(empty($formData['email'])) { // assuming your form has an email field which is named 'email'
        return;
    }

    $user = get_user_by('email', $formData['email']);

    if(!$user) {
        return;
    }

    $ldCourseId = 456; // Replace with your actual course ID
    ld_update_course_access( $user->ID, $ldCourseId );
}