I’m not sure where the #7 is coming from here as I don’t have that selected as an option. In any case, this seems to be as close as I can get. I would then just edit this chord suffix to appear the way you prefer.
I typed the same thing in the popover and got a similar result, but without parentheses. (In Engraving Options, I chose “do not use parentheses” and have the triangle selected: Cm∆.)
If it’s a bug, then please fix it, Dorico team!
BTW, when I double-click the symbol after adding it, the popover reads “Dm6/9add#7”.