Filter the default field value

pewc_default_field_value.php

<?php
/**
 * Enable a default value for checkbox groups.
 * Your default value must be an array.
 * You'll need to change the value of the $field_id to match your own Field ID
 * @param	$value			The current value of the field
 * @param $id					The field ID in the format pewc_group_888_999
 * @param $field			The field object
 * @param	$posted 		The $_POST object
 */
function prefix_set_field_value_by_url_param( $value, $id, $field, $posted ) {

	// Change this to the ID of the field you are setting a default for
	$field_id = 8018;

	// Don't overwrite a value that's already there
	if( isset( $_GET['my_param'] ) && $field_id == $field['field_id'] ) {
		$value = esc_attr( $_GET['my_param'] );
	}

	return $value;

}
add_filter( 'pewc_default_field_value', 'prefix_set_field_value_by_url_param', 10, 4 );