Start by adding a cylinder with 7 vertices.
Switch to edit mode Tab and select the 7 vertical edges of the cylinder.
Bevel the selected edges either by using the Bevel tool or by using the shortcut CTRL+B. Press the mouse button and drag until you have the desired beveling or adjust the width option in the last operation panel. You can use Numpad 7 for a top down view.
Use the Loop Cut tool or press CTRL+R while pointing with the mouse between the beveled areas. This will add loop cuts that we can indent slightly to match the reference image. Repeat this until all areas in between have the loop cuts.
Select the created loop cuts and scale them along the X and Y axis (S and SHIFT+Z).
Select the vertical edges of the beveling and use the Offset Edge Loop Cut to add supporting edge loops.
Close the top (and bottom) cap of the cylinder by selecting the edge loop, extruding (E), scaling inwards (S, SHIFT+Z) and filling the cap (F).
Add a bevel for the transition to the cap.
Finally set shading to smooth and add a subdivision modifier.