Thank you!
Login (replace URL, PORT, USER, PASS)
https://URL:PORT/webapi/auth.cgi?api=SYNO.API.Auth&method=Login&version=1&account=USER&passwd=PASS
ActiveBackup
https://URL:PORT/webapi/entry.cgi?api=SYNO.ActiveBackup.Activation&method=get&version=1
Set ActivationStatus (use your SERIALNUMBER)
https://URL:PORT/webapi/entry.cgi?api=SYNO.ActiveBackup.Activation&method=set&version=1&activated=true&serial_number="SERIALNUMBER"