Sorry, my bad, now I will try to fix my mistake.
For making flang on a top of cone You can switch to part enviroment and make a revolwe (flatness is unavilable in this case) or use tool to wrapping shore in sheet metal enviroment (which is shown in the attached screenshot) but in this case you have limited choice of the shape of the bend.
Easiest way is to do it as a part and just revolve the cross section - thats only one step.
The reason you can't add a flange around a curved edge is probably because that isn't a true bend - it relies on metal deformation.
Should still be possible though, and a good enhancement request.
@Wollverine - you can use the hem command with the Open and Bend Outside options to create the bend.
Then you can use a contour flange to create the flat.
Would need to set the angle of the hem according to the cone angle - so a bit of calculation there.
Just realised contour flange will work if you include the curve of the bend.
The cone height would have to be adjusted accordingly, but you can do that by creating the full contour, then using that to position the sketches for the lofted flange.