@EnenDaveyBoy Thanks for reaching. Please confirm if my understanding is correct that you want to configure the custom domain on your function app so you can call the function endpoint using your custom domain rather than the azure domain of your function app.
If it is the above case then you need to configure the existing custom DNS name to your azure function. You can refer to this app service document for more details. This holds valid for function app as well. You can also refer to this document for the configuration.