To practise this shot, ... Open console ~ in main menu and type OPEN BRIDGE When map is running, open console (~) again and type CLASS G Now you have the M203 grenade launcher.
You can follow you 203 nade's flying path by opening your console again and type BIND P VIEWCLASS PROJECTILE By shooting your nadelauncher now, and by pressing P after hitting the trigger, the 203 nade will be followed. You can apply any key to this action, I chose P voor Projectile. This will also work with rockets.
First stand against the wooden board and aim directly in the middle of it.
Then turn around and aim next to the tower,... just let your HUD (yellow dot) lay and relax against right side of tower, next to that point. (Also shown in next picture where it's zoomed in).
After laying your HUD against the towers point, move it up as shown on this picture. If you look closely you see me using all 3 points. Indicated with arrows.
After hitting the trigger, this will be your final result. Keep practising!