You are on the right way, but repetierserver has no password at all. Only way to become it is with sudo.
Check in user manual in "Advanced Setup" we describe how to add external calls to menu entries. This also includes sudo definitions for the user. On the other side if you start it from there it is already repetierserver. The sudo is only if you need to run it as root in this case. But this goes for any user to run a special command as a different user.
