Hi. I would like to create a contact for a different user, without knowing his/her password.
I have an api user, that has admin rights and can create users.
But is it possible to create a contact, that a different user can see in her/his address book?
As a Nextcloud administrator you cannot see any user specific file, except you are using the existing function to mimic a user in the GUI. If you want to write to the users personal address book by using an application you need to set-up an app specific password for the user.
Alternatively you can create an address book which you share with the user. The user will then be able to see the contact details and also edit it, if you grant edit rights to him/her.