For anyone interested, I guess one way to be sure is to use custom tags with the identifier attached. Its messy and I'm not even sure if it's necessary but it's a workaround:
Call getUsername(player.account)
+ function: On "getUsername"
-> ajax: Send AJAX request with tag "getUsername_" & function.Param(0))
+ ajax: On any completed
-> json: Load content from ajax.LastData
//tokenize the tag to get the tag name
----+ System: tokenat(ajax.Tag,0,"_") = "getUsername"
//tokenize the tag again to get the account name
--------+ player: account = tokenat(ajax.Tag,1,"_")
---------> player: Set account to json.At("data.username")