Adding a voice mid-measure

I'm trying to introduce a second voice mid-measure. This is Shostakovitch, op87, A major prelude, measure 26.

My problem is introducing a second voice mid-measure. I need to attach an A dotted quarter note to the DEF sequence. I put in the DEF sequence then click back on the D, go into note insert mode, and then click on the second voice button "2". But the program moves the insertion point all the way back to the first note of the measure in measure 26 and "wants" to start voice 2 notes there.

Might someone have a suggestion? Haven't been able to figure this out; I assume it is possible to do. Many thanks in advance! Malcolm