Hey @Nelly247, welcome to the community!

When a user signs up, they should be added to a List in Klaviyo.  You can then create a Flow based on a List subscription, which automatically sends the email. 

In this first email, you can either add the promo code if it’s a “static coupon code” - which just simply means it never changes from user to user. You simply add the static coupon code as you would with any text or graphic design in a template.

Or, you can send them a “unique coupon code” based on rules you set (percentage, fixed dollar amount off, free shipping, minimum purchase requirements, limited to certain products, expiration date, etc). 

Typically, this first email is part of a series of emails you would want to send when someone first signs up - so this is commonly called the “Welcome Series” or “Welcome Flow.”  You can see Klaviyo’s excellent guide on this (linked below) to help you get started.

Here’s some Klaviyo Documentation that might help:

• Getting Started Handbook - Guide to Creating a Welcome Series
• Unique Coupon Codes for Shopify (if you us Shopify)

Thank you! This is helpful.