Sandeep Arora has some scripts for us:
To create a distributed availability group, you need two availability groups (AG) each with its own listener, which you then combine.In this case, one availability group is on-premises and the other needs to be created in Microsoft Azure. This example doesn’t cover all of the details like creating an extended network setup between on-premises network and Azure or joining Azure active directory domain services to and on-premises forest; instead, it highlights the key requirements for setting up the availability group in Azure and then configuring the distributed AG between the on-premises availability group (represented as AOAG-1) and the Azure availability group (represented as AOAG-2).
Click through for the preparations you need in place and a set of scripts to do the work.