Hey! I have experience with Shopify, as I'm an owner of a store as well. I can either help and guide you to tell you what you need to do to get the result you want, or I can set it up for you in maximum 2 hours. Usually it should take an hour or less. I have experience with campaigns and email marketing as well.
First, I'd need your login information or to create an admin profile for me, from which I'd have access to your store.
What I would do is instal an app necessary for this task.
Create a profile for your store.
Create a coupon, unless it's already created.
Create email sign up pop up and instal it, unless already created.
Create a function, which sends out email with coupon code upon subscription.
Test the popup subscription to make sure emails are being sent.
Or I can send you steps on how to do this yourself, whichever you prefer.
You can contact me via email, or this website.