Programmatically adding User Capabilities to Azure DevOps Agents

I am automating the process by which we keep our build agent up to date. The basic process is to use a fork of the standard Microsoft Azure DevOps Pipeline agent that has the additional code included we need, notably Biztalk.

Once I have the Packer created VM up and running, I need to install the agent. This is well document, just run .\config.cmd –help for details. However, there is no option to add user capabilities to the agent.

I know I could set them via environment variables, but I don’t want the same user capabilities on each agent on a VM (we use multiple agents on a single VM).

There was no documented Azure DevOps API I could find to add capabilities, but a bit of hacking around with Chrome Dev tools and Postman got me a solution, which I have provided a GIST