Every newsletter in Audience comes with its own subscriber signup form.
In the newsletter screen under the "Subscriber Forms Option"
You can use the API and AudienceId provided here to make a POST call from any form and once you send email, we'll add it to the subscriber list.
We also provide a sample form, you can use as a starting point.
This will create the sign up form.
By default, Audience will send the opt-in email to anyone that signs up.
Once the email has verified they want to receive the newsletters, Audience will be able to include them in the sending of the newsletters.