I need to get the value of the personal property bossusername of the user test (as in the screenshot).

I found an api method to get personal properties of users:
[GET] {url}/rest/api/latest/user/properties/bossusername?username=test
but this method finds nothing.
Then, I decided to create another user-property using the API method:
[PUT] {url}/rest/api/latest/user/properties/newprop?username=test
JSON BODY: "newprop"
now i can get the value of this property via API, but it is not visible through the UI.
It turns out these are two different custom properties that do not see each other. Now I need to add a property through the API so that it can be seen in the GUI.