Azure DevOps - Azure library variables to TF variables

I want to map Azure Library variables to TF variables. I am unable to do so, output.tf doesn't display the variables when I try to pass them through the pipeline. Anyone has done this? Thanks RK.

eg: 

TF_VAR_x = $x

TF_VAR_y = $y

output.tf

output "X"{

value = X

}