Register Helper fields that also use the default WordPress meta field names for "First name" and "Last name".

pmpro-rh-for-legacy-brands.php

<?php
function my_add_pmpro_RH_fields()
{
    //require PMPro and PMPro Register Helper
    if (!defined('PMPRO_VERSION') || !defined('PMPRORH_VERSION'))
        return;

    $fields = array();
    $fields[] = new PMProRH_Field(
        "first_name",                                   // Input name, will also be used as the key to access as user meta
        "text",                                         // The type of field to use
        array(
            "label"             => "First name",        // Label to use for the field when displaying it
            "size"              => 40,                  // Size of the Input field
            "class"             => "pmpro_rh_names",    // Include in the user's profile (true | false | 'only' | 'only_admin')
            "profile"           => false,               // Include in the user's profile
            "memberslistcsv"    => false,               // Let field be included in "Member List" CSV export (true | false)
            "addmember"         => false,               // Used if the "Add Member Admin" add-on is present (true | false)
            "required"          => false                // Make this field required (true | false)
        )
    );

    $fields[] = new PMProRH_Field(
        "last_name",                                    // Input name, will also be used as the key to access as user meta
        "text",                                         // The type of field to use
        array(
            "label"             => "Last name",         // Label to use for the field when displaying it
            "size"              => 40,                  // Size of the Input field
            "class"             => "pmpro_rh_names",    // Custom CSS class to add (your choice)
            "profile"           => false,               // Include in the user's profile (true | false | 'only' | 'only_admin')
            "memberslistcsv"    => false,               // Let field be included in "Member List" CSV export (true | false)
            "addmember"         => false,               // Used if the "Add Member Admin" add-on is present (true | false)
            "required"          => false                // Make this field required (true | false)
        )
    );

    $fields[] = new PMProRH_Field(
        "organization_name",                            // Input name, will also be used as the key to access as user meta
        "text",                                         // The type of field to use
        array(
            "label"             => "Organization name", // Label to use for the field when displaying it
            "size"              => 40,                  // Size of the Input field
            "class"             => "pmpro_rh_names",    // Custom CSS class to add (your choice)
            "profile"           => true,                // Include in the user's profile (true | false | 'only' | 'only_admin')
            "memberslistcsv"    => true,                // Let field be included in "Member List" CSV export (true | false)
            "addmember"         => true,                // Used if the "Add Member Admin" add-on is present (true | false)
            "required"          => true                 // Make this field required (true | false)
        )
    );

    //Add the fields into a new checkout_boxes are of the checkout page
    foreach($fields as $field)
    {
        // Add the field(s) to the checkout page.
        pmprorh_add_registration_field(
            "checkout_boxes", // location on checkout page
            $field            // PMProRH_Field object
        );
    }
}
add_action("init", "my_add_pmpro_RH_fields");