I just clicked to mark it, then clicked again to cancel the mark, and then clicked on a grid at will. What a surprise (o﹃o) Fortunately, it was only a master level level, and the probability of hitting thunder the second time was still small.
Dreaming back to old times
Only the first clicked position must not be a thunder. If you want the first click not to be a thunder, re-open the first place where the flag is placed. Do not click anything else.