How can I add SMS opt-in to my password page?

I want to replace and/or add sms opt in on my Shopify password page. How can I do this ?

Ah thank you! What part of the password.liquid field should i put the api key? And is that all I’ll have to add ?

Oh i was asking where specifically in password.liquid should I add the sms api key to add a sms form on the password page ?