To create a hairpin at any point (e.g. the middle of a note value) make sure the caret is on and you are in the right place in the bar (right arrow along the grid). Then press Shift + D and enter the dynamic at that point.
A suggestion:
It would seem to me that the best way to ensure that the cellos and basses start the hairpin on the third beat of the bar would be to replace the dot on the minim by a tied crotchet. Then the notation is unambiguous and the placing of the hairpin even easier.
David
I agree - this is a case where “a hairpin starting in the middle of a note” can look unambiguous in the full score where the notes on other staves show the rhythm clearly, but it is ambiguous in the player’s part, which is more important for human performance!