it works great, here is the way (important! no space between letters) and momentely choose a administrator password with only letters and numbers and once done you can change it.
step by step
1: https://webaddress:port/webapi/auth.cgi?api=SYNO.API.Auth&method=Login&version=1&account=administrator&passwd=password
2: https://webaddress:port/webapi/entry.cgi?api=SYNO.ActiveBackup.Activation&method=get&version=1
3: https://webaddress:port/webapi/entry.cgi?api=SYNO.ActiveBackup.Activation&method=set&version=1&Activé=true&serial_number="serial number of your xpenology"
Enjoy