gistfile1.php

<?php
 
function searchwp_include_only_tax_term( $ids, $engine, $terms ) {
	// if mycat is the GET variable in use, we want to limit to a specific custom taxonomy term
	if( isset( $_GET['mycat'] ) ) {
		$my_taxonomy 	= 'my_taxonomy'; // taxonomy name
		$my_term		= absint( $_GET['mycat'] ); // taxonomy term as defined by form submission
		$args 			= array( $my_taxonomy => $my_term, 'fields' => 'ids' );
  		$ids 			= get_posts( $args );
	}
	return $ids;
}
 
add_filter( 'searchwp_include', 'searchwp_include_only_tax_term', 10, 3 );