If you just want to draw a circle in the middle of a rectangle, then the way Kevin provided should help. Another possible way for your reference:
1. Invoke Center Diameter Circle command and hover the mouse over the mid point of one edge of the rectangle - MidPoint shows, and then move the mouse to the center - Reference line shows.


2. Continue to move the mouse to the mid point of the other edge - Midpoint shows. And move the mouse towards the center - Reference line shows


3. When the mouse is at the center, two Reference line will show - Now click to specify the center and draw the circle.

Hope that's what you want. If not, please show us snapshot.
Thanks,
Macro Liu (Fusion 360 QA)