How to add an SMS popup to collect phone numbers and automatically add to my SMS list

[Erica00]

Is there an app or two-app combination to ask for customers' phone numbers and automatically add them to my SMS marketing service? 

I'm planning to offer discounts in return for phone numbers. This whole process needs to be automated. 

I tried different email marketing services' popups, but they slow down my store one way or another. Any suggestions?