I have a service in the same project named mpc-api - how do I set the env variable for another service to load the static URL of this service?

e.g. I want the env variable to be equal https://mpa-api-mytest-pr-12.up.railway.app - to do so do I set the value of the env var to: RAILWAY_{mpc-api}_STATIC_URL - or `RAILWAY_mpc-api_STATIC_URL?

RAILWAY_{mpc-api}_STATIC_URL returned nothing (replaced with empty string).