Hi @Steve_Parry,
You have to select the passage including the key signature* (with an extra command+click to select it, for example). Here below a short video showing it. You can use the calculation inside the Transpose dialogue, and after clicking Apply, you see that Dorico calculate an Augmented Unison Up. Then click on ok and it works very well:
(*) Tip: if the current key signature is C-major or A-minor, they will appear only as a signpost. Make the signposts visible and select them together with the passage, and proceed as shown ion the video.