Installing integration fails with trail Hunter Hydrawise account

Soon a hunter sprinkler system will be installed at our place. To test the integration with HA, I have created a trial account. However, I cannot get the HA Hunter Hydrawise integration to work.

During installation when the integration asks for the API key, I get back as error message: Please provide the username and password for your Hydrawise cloud account I am sure I have provided the correct API key.

When I test the REST API directly using curl, I get the following error: Controller must have a serial number. Please attach a valid serial number to this controller.
Since I do not have a valid serial number yet, I am unable to attach a serial number to the controller.

Could it be that I can’t get the Hunter Hydrawise integration to work because the serial number is missing?