You don’t say how exactly you want the trace to extend out from the box, but you probably want to multiply your offset vector by the box’s rotation converted to a vector before you add it to the start location. That way the offset vector ‘turns’ along with the object. Also people usually use the forward vector for line traces not the up vector, unless you’re trying to see what’s above you and not in front of you.
Finally, here’s a good tutorial on youtube that shows you what I mean by this extra multiplication: Unreal Engine 4 Vector Tutorial 3 - Working with vectors - YouTube
Good luck!