Set default field values for PMPro User Fields.

pmpro-user-fields-default-value.php

<?php
/**
 * Set default values for PMPro user fields.
 *
 * Supported field types: text, textarea, checkbox, radio, select, select2, multiselect, number, and date.
 * Can support readonly and hidden fields, however, they will not be editable by the user.
 *
 * 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_user_fields_default_value() {
	global $pmpro_user_fields;

	// Set default values for the fields. Format: 'field_name' => 'default value'
	$default_values = array(
		'example_hidden'      => ( isset( $_GET['cat'] ) ? sanitize_text_field( $_GET['cat'] ) : '',
	);

	// Loop through the fields and set the default values.
	foreach ( $pmpro_user_fields as $field_group => $fields ) {
		foreach ( $fields as $field ) {
			// Check if a default value was specified for the field name and set the field value.
			if ( isset( $default_values[ $field->name ] ) ) {
				$field->value = esc_attr( $default_values[ $field->name ] );
			}
		}
	}
}
add_action( 'init', 'my_pmpro_user_fields_default_value' );