Hey Matt - hope all is with you, it's been a while!
The TenantID should not be read-only.
Does the user you are authenticating with have access to that Tenant? i.e. are you able to sign into Method normally to that Tenant with the user name and password/sessionid you are specifying in your request?
Also, are you sure that the account you are doing this to is a multi-tenant account? i.e. if you have a sandbox account that you are using to test things out for your client, you should make sure that the sandbox account has been converted to multi-tenancy as well (you need to email us so that we can run the conversion).
Cheers,
Paul