If you haven't already, you'll need to sign up for a Klaviyo account.
To collect phone numbers for SMS marketing in Klaviyo, you'll first have to make sure your Klaviyo account is setup to accept them.
Setting up your Klaviyo API key
Navigate to the API Keys screen, by clicking your profile picture and selecting "Account". Next, click 'Settings' and then "API Keys" in the dropdown. (or click this shortcut)
Paste in the Private API Key on your settings page under "Integrations"
You're all set!
💡 Tip: You can now head to the opt-in form support article to add/complete your opt-in form.