Creating a proof of your 3D content in the SketchFab player is as simple as copying the embed link and creating a new proof using the website URL option.
To create a SketchFab 3D player proof
Step 1
In SketchFab, click Embed.
Step 2
In the Embed viewer, select Autostart and any additional options. Click on the embed code to copy it to your clipboard.
Example:

Step 3
Paste into a text editor the embed code and look for the src="......" link.
It will look something like this:
src="https://sketchfab.com/models/14c724212a2c469fa6cd051fb049aa51/embed?autostart=1"
Copy only the content between the quote marks.
Step 4
In PageProof, click the file dropper icon on your dashboard and select Website URL.

Paste the URL link you copied and follow the usual steps to send your proof.