If you would like to use “Coming Soon” page first you have to create a page in Pages → Add New with “Coming Soon” template in “Page Attributes”.
Then you have to set “Coming Soon” page as a homepage in Settings → Reading
To configure the coming soon sign up form, login to you https://mailchimp.com account and go to Campaigns -> Create Campaign -> Signup Form -> Embedded form, choose your audience and click Begin. You will be presented with configuration screen where’s a bunch of code in the Copy/Paste Section.
I’ve marked on screenshot below the parts you need to extract to put in Page Settings in Listeo