How to get Artifact Signing working with personal account

My old code signing provider tripled their prices, so I'm trying to switch to Azure Artifact Signing. I followed the getting started instructions, got my identity validation completed, and followed the instructions for using signtool (which is what I used with my old provider.)

It looked like everything was ready to go, but when I tried to sign a file, when I tried to log in, I first got an error that I don't remember the exact wording of, but the gist of it was that my Microsoft account (which is a personal account) needed to be invited to be a guest in the organization (even though it was the same Microsoft account that I had used to set everything up.) So I sent myself an invitation. (Looking at the email, it's saying it's an invitation to join "Default Directory".) I clicked the "Accept Invitation" link in the email, and it sort of looked like it had worked.

So I tried signing the file again, and now I'm getting the following error:

"Selected user account does not exist in tenant 'Microsoft Services' and cannot access the application '04b07795-8ddb-461a-bbee-02f9e1bf7b46' in that tenant. The account needs to be added as an external user in the tenant first. Please use a different account."

I've searched and poked all around in the portal and can't seem to find any way to add my account as an external user to "Microsoft Services."

The only Microsoft account I have is the personal account I am using. Is it even possible to use Artifact Signing using this account? If so, can anyone tell me what my next step is?