Create a link to apply discount automatically
Shopify provides the mechanism of shareable discount links. In a nutshell, when your customers click such a link, then they are redirected to your store homepage, and the discount is automatically applied at checkout.
When you use an email marketing service other than Mailchimp, then Personal Discount gives you a code snippet that already works like the shareable discount link.
If you use Mailchimp, then you'll need to prepare a sharable discount link manually. To do the trick you need to construct the link in your email template as follows:
https://{shop_name}.myshopify.com/discount/*|DSCNT_CODE|* where {shop_name} should be replaced with your shop name.
Email template with sharable discount link for Shopify
A final email will look for example like this:
Discount code in Mailchimp displayed as a sharable link
When the link is clicked then the discount will be automatically applied at checkout:
Sharable discount link to Shopify checkout