Google Directory Api: Get Users without G Suite

This may seem like a daft question but how do I return a list of users with https://developers.google.com/admin-sdk/directory/v1/reference/users/list I have tried all kinds of parameters with no luck in the Api Explorer. I am admin on the account I am trying with. In customerId I specify my_customer but I get Invalid Input. Not sure about the prerequisite: A G Suite domain with API access enabled. Is that really necessary? Kind regards