Create your own signup form in Ecomail and add it to your Shopify website.
You can do this in a selected contact list in the Signup forms tab. View this article to find out how to create and set up a new subscription form.
Once the form is created and you want to put it on your website, just press "Use this form" in the upper left corner and you will see the code (script) that you need to embed into your page. The script needs to be placed in the code of your Shopify account.
You can add the script to the code of your website by going to Admin > Online Store > Themes > Action drop-down > Edit Code.
To insert a pop-up form or a slide-out bar, open theme.liquid and insert the script anywhere between the tags <body> and </body>.
A static form can be inserted anywhere based on where you want it to be displayed on a specific page. There are available templates on the left panel, so you'll be guided accordingly in which part of the website you want the code to appear.
No worries if you make mistakes, Shopify is ready for that. You can revert the code that you mistakenly placed by following the guide available here.