Add First Name field to checkout, after username

my_pmpro_add_firstname_to_checkout.php

/**
 * Add first_name field to checkout, after username
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_add_firstname_to_checkout() {
	
    // Don't break if PMPro is out of date or not loaded.
    if(!function_exists( 'pmpro_add_user_field' )) {
        return false;
    }

    // Show fields on initial checkout only
    if( is_user_logged_in() ) {
        return false;
    }

    // This is where our fields code will go.
    $fields = array();

    // PMPRo_Field object for first_name
    $fields[] = new PMPro_Field(
        "first_name",
        "text",
        array(
            "label" => "First Name", 
            "profile" => true,
            "required" => true,
        )
    );

    foreach($fields as $field) {
        pmpro_add_user_field(
            'after_username',   // Location of fields on checkout page
            $field  // PMPRo_Field object
        );
    }
    
}
add_action( 'init', 'my_pmpro_add_firstname_to_checkout' );