ArtStation supports the Marmoset Viewer (.mview) format natively. In Marmoset Toolbag 2, click on File -> Viewer Export to start the process. 

Currently, ArtStation limits the viewer file to 15MB on free accounts. This is to ensure fast load times and allow the viewer to work across as many devices as possible. Pro users can upload up to 50MB to showcase their work in all its glory.

For more information about the Marmoset Viewer, please visit Marmoset's official documentation here.