Agisoft Metashape is a stand-alone software product that performs photogrammetric processing of digital images and generates 3D spatial data to be used in GIS applications, cultural heritage documentation, and visual effects production as well as for indirect measurements of objects of various scales.
Before you export your model, it is best to make sure the file size will fit your Sketchfab membership. If you're using the free Basic membership, your file size limit is 50MB. Generally, you'll want to decimate your mesh to 500,000 faces (as an OBJ) and then re-texture it.
Once your model is ready to go, click File > Upload Data and you'll see the pop-up below.
Select Sketchfab from the left side of the menu and choose your model's Source Data from the drop-down: Sparse Cloud, Dense Cloud, or Model.
Add your model's title and description, as well as your Sketchfab API key.