I would like to apply a chamfer to the highlighted edge.
But every time I apply the chamfer it runs along the entire circumference.
How do I apply it only to one edge.
The easy way is to create the chamfer on the edge before the corners of the extrude are rounded. Picture and model tree for clarification.
Another option is to use the chain option:
Select the edge, then holding shift select the edge again.
Additional adjacent edges can also be added while holding shift.