Need Help Using Graph Api while uploading a image in Instant Form using "upload_gated_file"

I need help with the Graph API when creating and uploading an instant form using this endpoint:
https://graph.facebook.com/v17.0/leadgen_forms.
Does the key "upload_gated_file": "YOUR_FILE" accept a string URL linking to a hosted file?
If so, can you provide an example of the format?
If not, is another key used for attaching images?
I want to ensure I'm using the correct key to create lead generation forms through api.