Here is a graphic that shows a Horn part. I added the key signature by saying 6s for 6 sharps. I understand that Dorico won’t understand a key reference, which I thought was ok.
I add a D into the horn part by pressing the letter D. It respells it as a Ebbb. Technically in the transposed key, this is correct. However, Db would make a lot more sense. Further more, hitting Alt+(arrow up or down) does not move the note.
In the same measure along the score (or offset by 1 measure), I tried tying two whole notes together. A vertical chord of F and D natural. I was able to input the first F and D nat., but in the next bar (in quard mode), I hit “T” for tie, the pressed F, and it gave me an F nat. I could not have it give me the usual F# per the key sig. I could add the D nat. just fine, and it added the tie. But I had to manually go over the F nat., and make it sharp, then re-add the tie.