There's a couple different options. One is that you can use a ConvertKit form on your "Coming soon" page (or use a ConvertKit landing page for that purpose). You'll then be capturing those email addresses into ConvertKit and be ready to go.
If you're not using a ConvertKit form on your website, but are using some other form system, you'll need to check the documentation for that form software to see if it can integrate with ConvertKit. If it doesn't, the easiest solution is to use the ConvertKit forms which you can create from within ConvertKit and then add to your website.