This is a known defect. Right, you could do it before but with the latest versions it has become impossible.
There's a following workaround, but you'll have to create two assignments instead of one.
As far as I understand, you need to schedule the next assignment
Mon: 6pm - 9 am (Tue)
Tue: 6pm - 9 am (Wed)
Wed: 6pm - 9 am (Thur)
Thur: 6pm - 9 am (Fr)
If yes, then you'll have to create two assignments to cover it:
1) Mon, Tue, Wed, Thur: from 6pm till 11.59pm
2) Tue, Wed, Thur, Fri: from 12am till 9am
This is what you'll get as a result
I was just actually able to do this on version 188.8.131.523. I had to go to the web site using the ipaddress and then it would let me create the entry overnight. It would not let me create the entry when I went to the DNS name that was setup. When I went to edit it though I could not save it. So if I needed to amke a change I had to delete the entry and recreate it.