You must be a Squad Leader to be able to request fire support from the Commander.

https://steamproxy.com/sharedfiles/filedetails/?id=1469812414

Once you have a target in your crosshairs, hold T and select Request Tactical Support from Commander

After you selected your target, press CAPS LOCK and you will see a blue icon with a timer.

Right Click on the icon and click the Green Check to confirm the target.

Once confirmed, the icon will turn gold and there will be a timer for the Commander to accept your request.