To create an Azure Function in the portal, click on + Create a resource and search for Function App.
All your functions, both new and previously created, are available through All Services > App Services for you to manage.
As always, you can continue to pick from a range of tooling options such as the cross platform CLI, Visual Studio IDE and VS Code to develop, debug or test locally, and deploy your function apps to your preferred region in Azure Government.
Note that Azure Functions in Government have a few variances from Azure commercial, read more about these variations.
Here are some resources to help you get up and running with Functions: