The standard technique for concatenation is to use macros, i.e at the top of the form tiddler (or in a separate tiddler tagged $:/tags/Macro) write
\define doid(dnr, cid) [DO#:$dnr$] CUS-$cid$
I'm making some assumption about the format described in your original post and also assuming that you know how to access the user set values.
In your form, where the new tiddler is created, you can now set the tiddler title to be the macrocall for doid.
Something along these lines at least. You might have to experiment a little but this is in the ballpark.
<:-)