How to set this boundary
 

Hi I want to set this boundary to the lower wall of my geometry how i can add it?

(dT/dy)=k(dT/dy)

where
dT/dy is the temperature gradient along the y-direction
k is the thermal conductivity

temperature gradient is varying along y axis, its simple in thermal distribution define k for y axis and leave it blank for others

Your answer is not clear i want to write udf for the temperature gradient?