Populate an Additional filed in class_app_shortcodes.php

Hi,

I have created an additional field in the admin. settings called Membership Number. This field now appears in the Confirmation page. All good so far!!!

In my Class_app_shortcodes.php I have declared the following variable

$membership_no= get_user_meta( $current_user->ID, 'membership_number', true );

I wish to populate the Additional Field with this value but the problem is how? I don't see the custom field in Class_app_shortcodes.php obviously as I guess it is created in the following: apply_filters( 'app_confirmation_fields', $ret );

Any help would be great!!

Thank you