Try this?

Cut out the area then place the board inside it then delete the board outline for the inserted board.

I tried and it didn't work... This is what I have for my layout and what is generated for the 3D render. It just eliminates the whole middle section completely.

The board outline for inserted board is still there and is a complete rectangle so that tells EasyEDA to cut out the whole area not just the incomplete slot that you require.

Assuming that you want to make a board as shown in your top photo:

Delete the board outline of the inserted board then add in a slot of the desired width and shape by drawing a track on the top or bottom layer then using right click convert to Solid Region set to CutOut.

Do not close the track. Leave a gap into which you manually place the stamp holes.

@andyfierman. I think I got it to work. Is this the correct way to add the cut line yourself?

"Is this the correct way to add the cut line yourself?"

I can't say for sure because I don't know exactly how you created it in your PCB.

It looks ok in your screenshots but you need to check the Gerbers using gerbv.

There are a few ways to do what you want to achieve but the technique I  described should work fine.

So when I generate gerber file it looks like this in EasyEDA

but when I open gerberview in JLCPCB it looks like this... :(

Nevermind, I figured out what I was doing wrong. Thanks for your help @andyfierman!

Check your Gerbers in gerbv (as described in the Tutorial) not just the JLCPCB viewer.

:)

@brad1379 So what did you do exactly do get to this. Searched everywhere online and i am actually building the same project and cant figure out how to make the cutout.