On the left is a tenon with the bottom rounded and the profile of the dovetail “cutter” on the top.
On the right, I selected the U-shaped edge of the tenon, got Follow Me and clicked on the triangle. There’ll be a few unneeded edges to erase on the shoulder of the piece but that’s all there is to creating it.
And thank you.
And here’s the socket. This assumes the two parts are in place as they’d be in the finished model. I made the two parts components so they won’t stick together.
Hide the rail getting the socket. Open the dovetailed rail component for editing. Select the end of the dovetail and the side surfaces along with their edges. Hit Edit>Copy or it’s keyboard shortcut. Close the component and unhide the other component.
Open the other component for editing. Hit Edit>Paste in place.
Erase the edge at the top of the dovetail socket and correct the face orientation so front (white) faces are out. Finished.