The 2 examples below rely on the fixed structure of the Activity column: -.
The first example uses the parse operator to assign values to 2 new columns: activityID and activityDesc.
SecurityEvent
| take 100
| project Activity
| parse Activity with activityID " - " activityDesc
This example uses the split operator to create an array of separate values
SecurityEvent
| take 100
| project Activity
| extend activityArr=split(Activity, " - ")
| project Activity , activityArr, activityId=activityArr[0]