If there are already on calls on your roster, your roster coordinator / manager can assign these to you as appropriate. Once they are signed off at month end, the payment will already be attached.
If there aren’t any on calls on the roster then you must contact Roster Support to request that these are placed on the roster. Roster Support will need the start and end times of the on calls, the days they occur and which grade of staff will work them
Guides for assigning on call time are on the E-rostering help pages on Connect