That will depend on your setup. Anytime you restrict an activity to run a on specific property or properties, you must include the appropriate at_property token in the call to Target.
On the activity overview page, you can see whether or not your activity is restricted to a specific property (see marker #1 below). If it is, you need to include the required at_property token (marker #2 below). You property token will be different than the one displayed in the image below.
The best way to think about the at_property token is as just another parameter (f.k.a. mbox parameter) in your request to target. When an activity is setup with a property, it is automatically includes parameter-level targeting to restrict entry to those requests that include the appropriate parameter.