Monosnap has a hidden option that allows automatically select a window for video recording.
To select a window for video recording:
1️⃣ Launch Monosnap
2️⃣ In the main menu, select Capture Area tool
3️⃣ Hold ⌥ key and click on a window with Capture Area tool
That is all! You’ll see the recording area in the size of the selected window.