Dorico treats things differently from Sibelius. Some design choices may be an advantage in one use case and a disadvantage in the other. In this case, the way that Dorico handles the interaction of dynamics in piano staves has some very specific reasons and uses.
Once you learn what to do, it’s not complicated. Just learn what to do, and it will work.
This seems to be one of those things that is simple see but very difficult to visualize. At least for me, I learn more effectively from videos.
If you want something independent, use ALT-Enter or OPT-Enter to close the popover. Simple principle. Works for multiple dynamics in different voices. Also works for other things like independent meters and independent barlines.
Here’s a 4-part fugue. How would you place any independent dynamics here without specifying which voice you want the dynamic to be applied to?
You can start input and change to the voice you want, or simply click a note in the voice you want to apply a dynamic to, input the dynamic into the popover, and close the popover while holding down Alt.
There must be a video on this subject.
Just… hold down Alt.
I’m sympathetic. Videos are helpful. But… just hold down Alt. 
Hi,
i prepared a video to help you on the steps that you need. I hope it can be of some help.
Have fun 
indipendet voice dynamic dorico tutorial
I attach also the file to test yourself:
indipendent voice dynamics.dorico (521.2 KB)
Entering dynamics into a voice
Dorico:
Enable Note Input > Choose a voice V (or create a new one Shift-V) > Shift-D > [enter dynamic] > Return > Alt/Opt-Enter
(all with the keyboard)
Edit: I can’t try now but you might be able to just select a note in any voice, and then press Alt/Opt-Return after entering the dynamic
Sibelius:
Select a note > Cmd-E > [enter dynamic] > Return > Select the desired voice in the Keypad
(using the mouse and keyboard)
Is this correct?
You are all incredibly kind, I started the video and then this came up and it stopped, “The media playback was aborted due to a corruption problem or because the media used features your browser did not support.” I must sleep on it, I’l 'keep the "Just hold Alt "in mind, I must get up early tomorrow. Good night to everyone.
Test Velocity piano 1.dorico (680.4 KB)
YES! Finally, I think I’ve got it.
Thanks again .
