A free and good alternative is Shopify Forms.
Creating a dedicated flow to people that convert on your popup form
To automatically send an email or sms to people that fill out your popup form, you can use Shopify Forms to create your popup form. And on the popup setting, you can use the “Add tag” field to assign a custom tag that should be added to any customers that fills out the forms.
Then in Spoks, you can create a new flow from scratch, and on the trigger select “Tags updated”. Then add a filter for the relevant tag on the trigger filter. Tag includes x.