Webservice link many to many entity

Hi all, I'm developing an app that use vte webservices, I be able to create a new Documents via webservice but I don't be able to link it to a sale order. I tried to add field like "parent_id", "parentid", "saleorderid", "RLparent_id" to the webservice request but nothing. Is it possible to do thath with standard ws? and how?

Unfortunately, at the moment, I didn't find a way to debug ws via eclipse debugger, so if someone know how to do that it could be easier to understand if this link is handled.

If is not possible with the standard ws, where I can edit ws file to handle this link? Is it correct to edit include\webservices\VtigerCRMObject.php inside the functions create and update? I saw VTE developer comments (// crmv@48267) in code.

Thank you