Hi Siow,
You have to do a few things here :
1. As per your provided data it seems that you are referring to " driving trigger " as start/schedule condition. So you can write badi codes to compute schedule/start condition. Please let me know if it's the same as per my understanding of the provided information by you.
2. You can then pass the username as one of the fields in the webservice, which can be tracked in the schedule/start condition badi. You can store the field read from the webservice either in a memory-id or in a static attribute of a z-class, which you can access in the processing code of your action profile( method_call ).
/Hasan