Use sp_addthreshold to create free-space thresholds.
See the Reference Manual: Procedures.
When the amount of free space on the segment falls below the threshold, an internal SAP ASE process executes the associated procedure. This process has the permissions of the user who created the threshold when he or she executed sp_addthreshold, less any permissions that have since been revoked.
Thresholds can execute a procedure in the same database, in another user database, in sybsystemprocs, or in master. They can also call a remote procedure on an Open Server. sp_addthreshold does not verify that the threshold procedure exists when you create the threshold.