Custom User Fields via REST API

Hi all
In our “old” checkmk 1.6 Version, we added Custom Field for Users, which we then could access via the WEBAPI.

In 2.1 we also added those fields, but im not able to see them via RESTAPI

I find the predefined fields like:

"pager_address"

or

"language": "string",

but i cant see my custom field.

What do i miss ?

Thx alot