Many account related endpoints require the use of a "tenant_id" parameter. Each account has a user on the Medium One platform of the format "t_<UUID>". The "<UUID>" is the tenant/account ID.
As SA, you can find this by going to Accounts and selecting an account. You can then find the account ID in the Account Information section:
When viewing a specific tenant as TA or TU, you can find the account ID in the URL.
Many account-related endpoints will also include the account ID (as key "tenant_id") in their response.