To add the stroke property to a circle you need to:
Instantiate a circle on stage.
Click the [+ add] button next to the circle's layer in the timeline.
Add the "Stroke" property.
Enter the value of the color you want your stroke to have on the displayed field.
Possibly tweak the stroke's width and opacity by adding these properties the same as before.
You can create stroke transitions over time like changing its color, its width or opacity by simply using different property values on different keyframes.
Note: You will notice that when enlarging the stroke of a shape, the bounding box of the shape crops the stroke. This is the default behavior of an SVG—stroke is centered on an object boundary, and the object bounding box hasn't changed.
If you go into Code Mode, find the SVG corresponding with this slice, and manually add
"style.overflow": "visible" to its timeline, the bounding box will adjust and will not crop the stroke anymore.