What, exactly, do you mean by dragged? Where was the ff before you dragged it?
The Manual is very clear, and yes, it does tell you about this:
Here an old post with a video I made, with the workflow, in case it could be helpful:
I see Alt-Return discussed in the 6.1 manual on entering dynamics through popovers. But the section in the 5.1 manual talking specifically about voice-specific dynamics does not.
The “ff” was about two notes away from the “mp” before I dragged it. By '“dragged” I mean while in write I clicked and held on the “ff” and moved it to the right. Thankfully, the end of the hairpin came with it.
It’s true that this section does not mention that Alt+Enter is needed for voice-specific dynamics; it suggests that all that’s needed is to have the caret active and the desired voice selected.
You need to scroll down to Related Links and click “Input methods for dynamics” and then scroll down to “Inputting dynamics with the popover” in order to find out that Alt+Return is required.
I think this could be made clearer.
I got it now, putting it into practice. Thanks for the last sentence.

