Passing a variable value to a hidden field in Frontend Form Submission

Thanks I did see that on the docs, but thought it was more of a general setting and I wondered about conflicts - e.g all user name fields as user_name that it would cause an issue in the post_meta table.

However, if I am understanding what you are saying is that I could have several fields e.g.user_name1, user_name2 etc and then declare them each in here with the same variable name in this case $user_name.

Thank you for this, I've literally read so much php/sass/jquery etc my eyes are going funny! Sometimes you can't see the wood, for the trees!

EDIT: I corrected a typo on the last 2 variables.

Here is how I am calling the form and that shows up exactly as expected
echo do_shortcode ("[mb_frontend_form id='request-membership-status' ]")

And here is one of the form fields to be populated with the variables
Screenshot

I solved it myself thank you, here is how I did it , for anyone else wishing to do this that has some level of skill but not a developer.

1. Created the Custom Field Types using the Metabox interface. Set them up exactly as I want, without the default value.
2. Clicked Get PHP Code and copied that code, minus the <?php tag into my functions.php
3. Added my PHP query to get the user details
`global $current_user;
get_currentuserinfo();
$user_name = $current_user->display_name;
$user_email = $current_user->user_email;
$user_first = $current_user->user_firstname;
$user_last = $current_user->user_lastname; `

4. added the 'std' => $user_name, etc etc to each field to show the correct variable
5. output the shortcode of the form <?php echo do_shortcode("[mb_frontend_form id='request-membership-status' ]") ?>
Job Done!!!
Here is the complete function I created so if anyone else wants to follow it they can see how it has to be set up as a working function for the MB Frontend Submission form.

add_filter( 'rwmb_meta_boxes', 'request_membership_status' );

function request_membership_status( $meta_boxes ) {
    $prefix = '';

    // your code to get the current user name here
	global $current_user;
	get_currentuserinfo();
    $user_name = $current_user->display_name;
	$user_email = $current_user->user_email;
	$user_first = $current_user->user_firstname;
	$user_last = $current_user->user_lastname;

    $meta_boxes[] = [
        'title'      => __( 'Request Membership Status', 'pinkequine-com' ),
        'id'         => 'request-membership-status',
        'post_types' => ['membership-request'],
        'fields'     => [
            [
                'name'          => __( 'First Name', 'pinkequine-com' ),
                'id'            => $prefix . 'user_first',
                'type'          => 'text',
                'admin_columns' => 'replace title',
                'std'			=> $user_first,
            ],
            [
                'name'          => __( 'Last Name', 'pinkequine-com' ),
                'id'            => $prefix . 'user_last',
                'type'          => 'text',
                'admin_columns' => 'after user_first',
				'std'			=> $user_last,
            ],
            [
                'name'          => __( 'Username', 'pinkequine-com' ),
                'id'            => $prefix . 'user_name',
                'type'          => 'text',
                'admin_columns' => 'after user_last',
				'std'			=> $user_name,
            ],
            [
                'name'          => __( 'Email', 'pinkequine-com' ),
                'id'            => $prefix . 'user_email',
                'type'          => 'text',
                'admin_columns' => 'after user_name',
				'std'			=> $user_email,
            ],
            [
                'name'              => __( 'Membership', 'pinkequine-com' ),
                'id'                => $prefix . 'checkbox_membership',
                'type'              => 'checkbox',
                'label_description' => __( 'Please add user Member to my profile', 'pinkequine-com' ),
                'required'          => true,
                'admin_columns'     => 'after user_email',
            ],
        ],
        'validation' => [
            'rules'    => [
                $prefix . 'checkbox_membership' => [
                    'required' => true,
                ],
            ],
            'messages' => [
                $prefix . 'checkbox_membership' => [
                    'required' => 'Please check the box to confirm your request',
                ],
            ],
        ],
    ];

    return $meta_boxes;
}