How to write CUSTOM metadata into JPEG with Python?

I tried

```
import piexif
exif_dict = {
                'uwi': myvalue1,
                'activity_type': myvalue2,
                'prediction': myvalue3,
                'url_current': myvalue4,
                'url_previous': mavalue5
            }
exif_bytes = piexif.dump(exif_dict)
with open(filename, "w") as fp:
    test_image.save(fp, "JPEG", exif=exif_bytes)

```

but see nothing in images with `XnView`. What am I doing wrong?

P.S. I don't need to write camera model, exposure and other stuff. I want to write my own custom metadata.