Does the kettle need to be constantly bubbling, meaning keeping the button down? Thanks

ב"ה Shalom The principle is that a utensil which needs koshering is to be placed in a pot on a heat source with bubbling hot water. However, when the utensil cannot fit into a pot, such as a countertop or sink, in the past , the bubbling water was achieved by placing a heated stone where boiling is to be poured, so that when the water is poured it does not go down in temperature. . (see Shulchan Aruch Orach Chaim 551:5) Today with electric kettles, the bubbling water is achieved by the heat element inside the kettles. If you can possibly keep the button down while pouring, that is fine. However, there is a Talmudic ruling of "Chmira sakanta m'issura" (Chullin 10a) which means that a danger is more severe than a prohibition. Therefore, if you are to do so you should exercise extreme caution.to prevent scalding. In my experience, if you take the kettle at the peak of its boiling before the automatic shut off, for the few seconds in which you pour the water it continues to bubble and that should suffice. There may be additional details relevant for the surface you are koshering. . All the best