Copy the number between /Manage/ and /CompanyEdit in the browser address bar. For example, in https://my.sandbox.billit.be/Manage/123456/CompanyEdit, the Party ID is 123456. Use your own account’s value, not this example.
This is the master account’s Registration ID, sent as the Party ID in API request headers. The screenshot above is from Billit’s Party ID guide.
Use the master account Registration ID, not the Registration ID of an entity or subaccount referenced in an API path. Billit requires the master account Party ID for every production API call and for KYC identification calls in both environments.