While the application is running, the window title should be listed in the relevant targeting dropdowns (assuming the window containing the dropdown was opened after the application was started).
The name of the process would normally be the name of the executable, minus ".exe". You can check that in the "Details" tab of task manager.
DCS' executable is named "DCS.exe", so to target the process, specifically, you would use "~DCS".
As noted in the "Command Screen" section of the documentation (press F1 while VoiceAttack has focus to open VoiceAttackHelp.pdf in your default PDF viewer), "~" instructs VoiceAttack to only search process names, thus making sure it doesn't spend unnecessary time searching window titles first (this also prevents unintentionally matching other windows, E.G. an explorer window opened to a folder named "DCS")
To target using the window title instead, you could use "^Digital Combat Simulator" ("^" instructs VoiceAttack to only search window titles).