Hello Kaj,
If you want the property to be present on the form during user creation, you need to add it in the Create tab for User object type in Adaxes Web Interface Configurator. After that, you need to refresh the Web Interface page using Ctrl+F5 to see the changes.
If the property still does not appear, please, post here or send us (support[at]adaxes.com) screenshots of the form you customized in Adaxes Web Interface Configurator and how the form looks in Adaxes Web Interface when you try to create a user account.