There isn't a way to upload your template to the Lead Gen Campaign gallery to use in the Email Sequence. And the process calls for showcasing a template, not a final cover that's already been built. Not only is it more labor-intensive, but it also eliminates the need to meet with the owner in the Pre-Production Meeting and delivery of their personalized covers. 

Technically, of course, you can create a cover set for your lead ahead of time and use the Draft Link instead of the Lead Gen template for Cover Preview: 



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