When I want to insert a quarter note on an offbeat, Dorico inserts two eighth notes with a tie. In the attached image, the top is what Dorico does, and the bottom is the sheet music I want Dorico to output.
[url][/Imgur: The magic of the Internet]
Hi rhdsmnd!
Welcome to the forum! 
Its quite easy: you have to use the »force duration« tool.
If you use this, Dorico overwrites the engraving standards which causes e.g. 2 tied eight notes.
So: bring your carret to the position you need, press shift-o, press 6, enter the note you need.
Good luck!
You might also find that once you continue inputting notes beyond that note in the bar, Dorico readjusts its notation and the note appears as a single quarter note again. Note grouping is context-dependent in Dorico, and if you have Dorico Pro, you can find the options that apply by default in each flow in Notation Options (Ctrl/Cmd-Shift-N) > Note Grouping. For instance, there are a couple of options about how to notate notes that come after a rest at the start of the bar and also how to handle syncopation.
Force Duration is really useful for corners where these options don’t produce what you want, or if you don’t have Dorico Pro, but it stops Dorico doing some of its magic if you e.g. later change the time signature: Dorico automatically correctly divides notes differently in 3/4 compared to 6/8 as long as those notes don’t have Force Duration active.
Sorry, forget my comment. Lillie is right.
First study the notation options and find out the best solutions for your project. You can change this options any time.
Use the force duration tool in the very end of your work where the algorythms of Dorico don’t show the desired result.
Pepinello,
IIRC in Write mode with the caret active, just typing “o” (w/o quotes) will activate Force Duration. SHIFT+O brings up the Ornaments Popover.
You are right, Derreck! 
Thanks so much everyone!