Can you put a curved slot in PCB in Altium 18?
Have you tried the "Convert" Method in AD? 

I often use this tool for such tasks. I don't know how this works in AD18, but I can explain for AD17: 

You can draw a rather thick curved track/line, select it and then use the "Tools/Convert/Create region from selected objects" function in the menu. 
After that, you can double click the region and set the "kind" to "board cutout".