How do I notate the highlighted measure?

Another option is like this.

Demo.mscz

I used insert note input mode to enter the cadenza notes. This results in an odd-length bar but never mind. Then I adjusted the beaming on the notes, selected the cadenza section and checked the "small" check box in the "chord" section of the inspector. I found I had to decrease the stretch of the bar a bit using "{" to get a similar spacing to that in the original. But quite close I think.