Didn't we already discuss this in invoke-restmethod to use vsphere rest apis ?

Or do you mean something else?

Btw, I do not have access to that API Reference.

I can have a look, but be aware that I currently don't have an active SRM setup of that version.
Send me a private mail (here or via my blog)

Thanks, I'm in

As far as I can tell, that API is not a REST API, but based on SOAP.

That eliminates the use of Invoke-RestMethod.

It looks as if the methods available after you do a Connect-SrmServer, is currently the only way to access the API.

You can use PS, just not the cmdlets that are intended for use with a REST API.
Xavier did a nice series on how to use the SRM API from PowerShell, see PowerCLI SRM module: How to use it