How to get a temperature value a one point and assign it to a boundary variable

i need to create a velocity for the moving mesh, this velocity takes the temperature value at (r=0) in the (2D axisymmetric model) and multiply it by a simple exponential function of r to give a velocity variable along a boundary. this seems to be a simple ting to do i a code, but comsol for some reason does not accept this and require me to define the temperature variable along the whole boundary.