There are a bunch of threads that ask a similar question. Start here: Tips for promoting a new free WordPress plugin
Honestly, I think it would be best that this thread title was renamed to something more specific to your particular situation: marketing a coming soon plugin. Instead of another topic on general plugin promotion which has been discussed quite a bit already.
You'll be up against some stiff competition. It's common for theme companies to offer some sort of free coming soon theme/plugin as a placeholder for customers of their full-fledged themes. Then there's dedicated businesses like SeedProd by @johnturner which is solely focused on building coming soon plugins.
How are you going to differentiate yourself? It's not immediately clear how your plugin does anything different than what's already available. Easier said than done, but I'd work on that.