If your goal is to grow your email list from Instagram like you said, then the zero-budget way is to put your store URL as your link in bio, then use the default Shopify newsletter pop-up with a 10% discount. That'll get some passive email opt-ins, but only a little.
If you really want to get email subscribers from Instagram, you should use organic Instagram incentives, like giveaways, which users love and drive huge engagement.
For example, run an Instagram giveaway and give people bonus entries if they join your email list. This works insanely well. There are Shopify apps to automate it for you, or you can do it yourself manually (it's just a lot more work).
When you're done, your new email subscribers will be created in Shopify as Customers with the 'accepts marketing' flag on so you can email them using Shopify Email or any email marketing service that syncs with Shopify.
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