To authenticate calls on the Development API of Webdashboard a few steps need to be taken. These are explained in a separate feature that can be found here:
This video explains in detail how to Add a user to Webdashboard through the Development API:
To Add a user to Webdashboard, navigate to the documentation page of Webdashboard.
On the top right click authorize and add your bearer token.
Then go to the Users section and op the AddUserToPortal section. Click on try it out and fill in the blanks.
As a result you will get the CURL you can use to automate the process and result of your action.