Hello Pau, 

If you are talking about setting a Fiscal position on the drop-down on the contact record, you just need to supply the id of the fiscal position with the call. 

Here is an example in PHP.

$common = ripcord::client("$url/xmlrpc/2/common");
$common->version();

$uid = $common->authenticate($db, $username, $password, array());

$models = ripcord::client("$url/xmlrpc/2/object");
$id = $models->execute_kw($db, $uid, $password,
    "res.partner", "create",
    array(
        array(
            "name" => "New Partner",
            "property_account_position_id" => 1
        )
    )
);

I hope this solves your issue,

Thanks,