Unfortunately you can't set it up that way. You can put in your description however that you will reduce the nightly rate by x% or $x after the first week. The only other option is to use the weekly and monthly discount settings, which is not really what you are looking for.
That really helps I'm glad I didn't pull my hair out trying to figure out someway to do something that can't be done. I have another question I'll ask you before I put another post. I was going to consider a shorter rental for the rest of October but I wasn't quite sure how to do that.
Is it possible to start a new listing for the same property?