predescribed displacement equals displacement value at certain point

hi guys,

I'm pretty new to Comsol and i'm trying to implement a certain kind of boundary condition. Perhaps there is an easy solution to this, but i can't figure it out. My Problem is that i want to use a constraint like u(1,1,1)-u = 0 for an edge setting. In words, the displacement on an edge is supposed to be the same as the displacement at a certain point.
No matter how i try this I get an error.

I really appreciate your help.

kind regards

Jan Kaul