Hi Sanjeev,
What type of Activity you are creating i.e. AB Testing or Multivariate or Experience Targeting as different different activities have different use cases ?.
If you want to create an offer for Audiences "Referred From Google" Then in that case , you can use Experience Targeting and there will be 1:1 mapping between the Experience and Audience and it will not be required to pass a separate mbox parameter.
Please do let us know in case of any more questions or queries.
Thanks & Regards
Parit Mittal