Setting ENV variables in XCode in not straighforward. I have broken down how to accomplish setting up an ENV variable in the following videos and code example.
Make a new conifuration file and set any ENV variables you need.
In the debug environment, use the debug.xcconfig file. Then, assign the value to something, in this case, SignUpUrl corresponds with the SIGN_UP_URL, which is set to localhost:4567/sign_up.
The sign_up_url variable is set to localhost:4567/signup.