functions.php

<?php

function my_searchwp_engine_customizations( $settings, $query ) {
	// print_r( $settings );    // uncomment this line to see the settings structure
	// print_r( $query );       // uncomment this line to see the query structure

	// Check for Title-only search
	if ( empty( $_REQUEST['searchwp_limit_to_title'] ) ) {
		return $settings;
	}

	// Return the Titles Only engine settings
	return SWP()->settings['engines']['titles_only']; // titles_only is the Supplemental Engine NAME (not label)
}

add_filter( 'searchwp_engine_settings_default', 'my_searchwp_engine_customizations', 10, 2 );