How to register Azure CSP sandbox?

Question

In my previous job I had Azure CSP Sandbox tenant for myself and there was basically no money cost involved. I don't know how it was done in details, but I was told it is a partner feature to be able to have multiple CSP Sandbox enviroments for Developers. I used E5 licenses there for testing Intune and Autopilot mainly.

Now with my current employer, they also have CSP deal with Microsoft but the use case seems not to be the same. They have partnership sponsor budged with Microsoft, but I was told that there can be only one tenant per subscription, and this partnership applies only to one subscription. Unfortunatelly no one knows in details, what and how it should be done. I also have an option to get new subscription and tenant opened for my self, but I wanted to low the cost for testing.