Dorico’s automatic labeling doesn’t have that kind of flexibility. You’ll have to manually edit the name. Unfortunately, the Edit Name dialog doesn’t support the {@flat@} token or the insertion of Music Text feature added in v4. So, you’ll have to copy/paste a flat symbol into the name.
These are the key options available in Dorico 4 for positioning of transpositions in staff labels - but as it looks like you’re not using Dorico’s automatic instrument numbering (?) you may well need to handle this manually as Craig suggests.