Expand the menu of the database for which you want to create a user and click «Create user»:
Fill out the form and click «Save»:
«Login» — the name of the database user. It always starts with an immutable prefix (green) containing hosting account namewhere the database is located.
«Password» — you need to manually specify the desired password or automatically generate a random one.
By default, the user you create is given all available rights, so they usually don't need to be changed. If necessary, the rights can be changed in the future. If you want to set them up right away, click «Privilege»: