Currently it is only possible to start a crawl with the enter key after typing the url.
I would suggest a "start crawl" button beside the url input field.

this button can change during crawling to a "stop" crawling button which is currently located under the url bar.

also I am not quite sure why there are 2 URL fields. maybe we could remove one?

The "start button" will also have the benefit we can restart the crawl… currently with the top url input, this is not possible.

It would simplify the UI a bit :)