Setting up a backend and separate Single Page Application (SPA) site

Hello

I am currently trying to set up the following on a Forge server.

I have created 2 sites called api.mysite.com and spa.mysite.com, I have created 2 DNS records to point my subdomains to my IP address but now I need to make requests from spa.mysite.com to the api. How do I set this up?

I have done it before but I can't remember! This is in part to document this for future use!