We had a long, fruitless argument about this same issue 3 weeks ago. The upshot is you will never get Dorico to think of tied notes as separate notes. It is one note with one duration. If you insist on entering each note as you see it on the page (with pitch first), then the best you can do for these ties is enter the notes separately and go back and tie them afterward.
Two tied dotted halves equals a dotted whole note. It’s faster, and a lot more fun to enter the whole thing at once.