Many forums support Sketchfab embeds. Some forums accept them natively, which means you will find a Sketchfab button in your post editor, and you just need to copy and paste the model URL of your model to embed it. See Embed on Web Platforms.
Sketchfab models support oEmbed. This means forums that use services like Embedly or Onebox can be configured to handle Sketchfab embeds. The Sketchfab Forum, hosted by Discourse, for example, uses Onebox.
Some forums support our iframes, so you can copy and paste the HTML embed code into the forum post. An example is Maxforums.
Some forums support BBCode embeds. It will display the thumbnail of your model with a link to view it in 3D. Here is an example on CG Society.
You can switch between HTML (iframe) and BBCode in a model's embed options.
If you want to add Sketchfab support to your forum, check out our plugins (and the oEmbed endpoint):