Perform the following steps:
- Navigate to the Custom domains blade of the Azure function app for which you would like to configure a domain:

- If you have created a custom domain from the Azure portal, you'll be prompted to choose hostnames, as shown in the App Service Domain blade. Click on OK:

- If you have chosen both of them, then that's it. All the work in integrating the function app and the custom domain is pretty much done for you by the Azure Management portal. You can view hostname integration here: