How can I deliver my own customer number for new contracts on signup?
For new registrations the customer data can be prefilled in the hosted order page or by subscriptionJS. If a customer number that has already been assigned is to be transferred with the registration of a customer, the field "Tag" can be used and prefilled. All fillable values are listed here in the "Hosted Customer Self-Service" section.
The tag field is designed as a technical field and is used to temporarily store the customer number or an external ID, but not as its final storage location. This is done using the ExternalCustomerId field, which is also searchable.
The intermediate step above day is necessary because the ExternalCustomerId must be unique and this is also forced in the database. An ExternalCustomerId that is already stored in the system causes orders to fail.
In order to prevent failed orders, the ID can be transferred via the "tag" parameter and transferred to the ExternalCustomerId after a successful order. The "tag" parameter can be queried after ordering via GET /api/v1/Customers/{id}. However, it is not visible in the UI, therefore a storage in another field is necessary. You can use PATCH /api/v1/Customers/{id} to store the information in another field.
For more information, see the billwerk REST API documentation.