Override the port of a mock service using a global variable

Hello everyone,

I have created a project with two mock services. In the second mock service I am calling the first mock service using it's url(I use the second service as a type of proxy). What I need is to "build" this url using the port of the first mock service, which I want to be overridden by a global variable maybe. 

Is that any way to do this?

Please help me, as I am stuck at this step for over a week and googling din't provide me any solution.

Thank you!