This will automatically be included with the payload with new Plex Movie libraries in 1.20.1 (on new items or once a user has refreshed their metadata).
It will show up in the Metadata element like this:
"Metadata": {
"librarySectionType": "movie",
...
...
...
"Guid": [
{
"id": "imdb://tt1231583"
},
{
"id": "tmdb://41733"
}
],
...
...
