Use Force Durations (shortcut O, the icon that looks like a G clamp in the note input toolbox at the left-hand edge of the window). With this switched on before you input your rhythm, Dorico will follow exactly the durations you specify. I recommend that you only engage Force Durations when you encounter a situation that Dorico doesn’t handle to your liking, however, as it will tie the program’s hands in other situations and reduces the effectiveness of its musical intelligence in adjusting the music as it later changes.