Hi Siow,
You have to do a few things here :
1.a 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.
1.b In case you are referring driving trigger as the code in the action processing ( method call , then you can directly move to step 2 without following 1.a ).
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
