Add html attributes to a custom PMPro user field created in settings.

add-html-attributes-to-pmpro-user-fields.php

<?php
/**
 * Add HTML attributes to PMPro user fields.
 *
 * 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 add_html_attributes_to_pmpro_user_fields( $field, $where) {
	if ( 'pet_name' === $field->name && 'text' === $field->type ) {
		$field->html_attributes = array(
			'placeholder' => 'Placeholder text here.',
		);
	}

	if ( 'number_of_pets' === $field->name && 'number' === $field->type ) {
		$field->html_attributes = array(
			'max' => 10,
		);
	}

	return $field;
}
add_filter( 'pmpro_add_user_field', 'add_html_attributes_to_pmpro_user_fields', 10, 2 );