Each Sketchfab membership offers different ways to customize the embedded viewer for your models. To remove the Sketchfab watermark, though, requires Premium or Enterprise membership.
If you've upgraded to Premium or Enterprise, you can remove the watermark for your models in two ways:
Toggling the options in the Embed viewer pop-up
Editing the model URL with embed parameters
Editing the embedded viewer options
If you click the Embed link below the model on your model page, you'll see the embed code itself and a list of options available per Sketchfab membership. Click Remove watermark and/or Remove watermark link under the Brand heading and the embed code there will change to suit your needs.
Editing the model URL with embed parameters
Adding embed parameters to your model's URL allows you to control different aspects of the model viewer. In order to use this method, you'll take your model URL and add the parameters to the end.
For example, let's pretend your model is the Cube Test and you want to add the embed parameter to remove the Sketchfab watermark. First, you'd get your model URL:
Then you'd add the embed parameter for the watermark (ui_watermark_link) to the end:
Setting an embed parameter to 0 or 1 determines how it functions.
With Premium or Enterprise membership, you can use any of the available embed parameters.