There are several ways
First I would look at is using a planishing hammer to stretch the metal in the area you want the curve shape. Then you can use a tipping wheel in a bead roller to create the line.
Second, you can make a hammer form for the flat part and then you can use an air hammer with a flat hammer anvil in it to run along the hammer form to set the line. The with a metal block on the back or up against an anvil you can use the air hammer to stretch the metal,to,create the shape you want in the panel.
Last method I would oook at is to take the material and form it by hand to get the desired curve. Then you can use a shrinker to shrink the metal and crate the flat area. To make the line between the curved and flat areas I would use a bead roller or hammer form.
Where do,you live, I have the equipment I recommend using.
You can look at YouTube and watch the videos from Lazze, and Wrey as a start for,how to shape metal