Mission plan with KML import on PC

You can draw the KML polygon or line string in Google Earth and save it as a KML. Them import it using the File Manager screen's Import button. 

You can also define flight areas (boundaries) on the Maps Made Easy site and sync those to your device.

https://support.dronesmadeeasy.com/hc/en-us/articles/360029963631-How-to-create-Boundaries