Add a popup window to a website on load. For signup to mailchimp & discount voucher.

We need to add a popup / layover window on a pre existing website on home page load. Will be used to sign up for mailchimp and to give clients a one time use 10% gift voucher. Site built on the Shopify platform, Ruby on rails. All work must be neatly coded & locations of code changed given. Work to remain our property on completion and payment. Thank you :)