monsterinsights_frontend_tracking_options_gtag_before_pageview.php

<?php 

add_filter( 'monsterinsights_frontend_tracking_options_gtag_before_pageview', function( $options, $type ) {
	$user_id = 999; // Replace with code to get actual user ID here
	$meta_key = 'my_meta_key'; // Replace with the user meta key here

	$meta_value = get_user_meta( $user_id, $meta_key );

	if ( $type === 'ua' ) {
		// If the customer is tracking for a UA property
		// they need to make sure the "index" they set up for that custom dimension
		// on their GA property matches this $index here:
		$index = 9;
		$options[ $index ] = $meta_value;
	} else {
		// For v4 properties, they need to make sure they have registered this
		// custom dimension's parameter name on their GA4 property
		$parameter_name = 'my_parameter_name';
		$options[ $parameter_name ] = $meta_value;
	}

	return $options;
}, 10, 2 );