$MHANDLE is the model handle of the model where the alarm was raised not the model handle of the user.
The following is the syntax of the assignticket utility from the docs:
assignticket Utility
The assignticket utility is used to populate the Trouble Ticket ID field of an alarm with the name of
the person to whom the ticket is assigned.
Run this utility using the following syntax:
assignticket modelhandle alarmid assignee [username]
modelhandle
Indicates the handle of the model where the alarm was raised.
alarmid
Indicates the ID of the alarm to which to write.
assignee
Indicates the name of the user to whom the ticket is assigned.
username
(Optional) Specifies the name of the CA Spectrum user account to use to connect to the
SpectroSERVER.