%%[
        SET @qs = "value|With|pipes"
        SET @URL = "WhateverMechanismYouUseToSetThis"

        IF INDEXOF(@URL, "testdomain.com") > 0 OR INDEXOF(@URL, "fakedomain.com") > 0 OR .... THEN

             SET @URL = Concat(@URL, @qs)
        ENDIF
        /* no need to do anything for else case */

  ]%%