Hello,
Azure Function App can be registered under (or linked with) API management in two ways:
Function App -> API Management -> Link API
API Management -> Add API -> Import function app.
Either options can be chosen over the portal. However, the key objective is to do the same (#1 or #2) with automation, i.e using Azure CLI.
With some discovery, looks like "az apim api import ... " would import the API with open API or swagger specification files, which doesn't meet the mentioned objective. Please provide if there are any references/samples available on executing on this requirement.
If there is no such option with Azure CLI, then the function API need to be registered as basic API (blank API) and create necessary operation. Perhaps, this would introduce more work.