To modify a gradual dynamic, select the hairpin and lengthen or shorten it using Shift-Alt-arrows.
Edit: for a messa di voce (<>), it’s a little more difficult. You’ll need to enter the crescendo, modify it to extend to beat 3, then enter note input mode, navigate the caret to beat 3, and input a decrescendo. Then you can modify that one as desired.
Dorico doesn’t give you as much fine-grain control over messa di voce marks. It only allows you to align the apex to a note. So in this case, you can’t just enter it as a single expression.
I think this is worthy of a feature request… it would be nice if Shift-Alt would move the center of the messa di voce according to the rhythmic grid, just as it does for other gradual dynamics. While dynamics should generally align to notes (which gives clarity to the performer), sometimes they don’t.