Make you shape in a graphic design software, like Photoshop (will produce a transparent PNG on export) or Sketch, who can produce a vector SVG.
You’re going to use this shape as a background image of your container, set to no-repeat.
However if your shape is a circle, you can cheat by doing it in Webflow. A circle is a square with huge rounded corners. Start with a div, put another div in it, give it a class, make it round, put your navbar in it and from there place your elements.