Honestly, not sure what you have done. But our SAMBA server installation script is not aksing for a single user input during install process. We setup a default SAMBA server configuration pointing to /mnt/dietpi_userdata as target directory and using SAMBA user dietpi. There is nothinng ask for user, password or target folder.
Probably you mixed the Server installation with setting ab a Client connection using dietpi-drive_manager. This indeed is asking for user/password. But these values are needed as client to be able to connect to a remote server. It has nothing to do with setting up a SAMBA server on DietPi.