Hi Michael,
Pix4D uses exiv2 to read the exif and xmp tags, so I would recommend you to use that. The tag would be Xmp.Camera.SunSensor. You can find the information about more XMP tags here: https://support.pix4d.com/hc/en-us/articles/360016450032-Specifications-of-xmpcamera-tags
The tags would need to be exact. You can also find some info here: Camera, Sun Irradiance and Sun angle in red text - #8 by Manuel_Cubero_Castan
If you need any help in tagging let me know.