Add camera rotation in addition to the HMD one (how to set pivot?)

I’m quite new of UE4, and I’m trying to add a rotation to the one given by the Vive HMD (this rotation should be relative to HMD position). Up to now, I didn’t manage to rotate with respect to the HMD relative position (it seems that rotation is wrt. character/pawn position).
As shown in the pictures below, in the Pawn Begin Play I add a Rotating Movement component to the Pawn Controller and in the Pawn Event Thick I try to set the HMD as pivot and to set a yaw rotation.

Can anyone help me?
Thank you