How do I rotate a quaternion with euler angles?

I think my mind is just in a knot at the moment, since I feel this should be fairly easy.

Right now I'm trying to rotate a quaternion by adding a few degrees to its y axis, so that its rotation is relative to different object.

Maybe I should try a different approach though; I've got the player moving around and following a target direction based on joystick input, and I like how it rotates, but I want to make the movement relative to the camera, instead of based on worldspace.

Here's the part I'm having trouble with:

         Vector3 targetDirection = new Vector3(horizontal, 0f, vertical);
         Quaternion targetRotation = Quaternion.LookRotation(targetDirection,Vector3.up);
 //        this is where I'd like to rotate the quaternion.
 //        targetRotation.eulerAngles.y += pRotation.eulerAngles.y;
         Quaternion newRotation = Quaternion.Lerp(rigidbody.rotation, targetRotation , turnSmoothing * Time.deltaTime);
 
         rigidbody.MoveRotation(newRotation);

Edit: total brainfart.

targetRotation.eulerAngles += new Vector3(0, parentRotation.eulerAngles.y, 0);

Was the solution. I'll delete the post if this is redundant.