I already created a hook to create an Supplier if anyone register at my wordpress site.

Now i need to set the Wordpress User-ID to the External ID in Odoo. But i don´t know how..

Or is there a list of all possible Key Values? 

I tried to add this but it might be not the right odoo-"key".
"external_id"=> new xmlrpcval($_REQUEST['external_id'], "string"),

Any Ideas on what key to use? Or is there a documentation for that?