You'll need Besiege in windowed mode and ShiftWindow for this, so download/install it.

Steps:

1. Open ShiftWindow
2. Open Besiege
3. Toggle the "Filename" box and press Grab.
4. Select Besiege.
5. Press Identify to confirm the window's identity.
6. Save your rule (Ctrl + S).
7. Press "Trigger it".

Note that this is for Windows and won't work/may not be necessary for another OS.