The easiest way is to plot one or two waypoint where you want to fly in Litchi, export it is as a csv file, convert it with Litchiutilities into a KMZ file, and upload the file into your controller.
Sounds a bit complicated, but after you did it a few times it is an easy workflow.
I also created a detailed step-by-step guide for this, maybe easier to follow:
DJI Air 3 for photogrammetry and Litchi
This tutorial explain how to do a complete field mapping, including how to map a field in dronelink first! For your case, only chapter 3 (exporting a litichi mission) to chapter 7 (opening the new waypoint mission) are relevant, so skip chapters 1 and 8.