Prior to following the steps below, you have created your eNewsletter at this point.

For Any Forms:
1. Go to Communities > Forms > Contact (go to other forms or FAF if needed)
2. Hover over the form name and click Website Info
3. Select option 5: Form Customization
4. Click Add-Ons link on the top
5. Scroll down and mark the eNewsletter you want available on the form for sign up
6. Click Save & Done

For eNewsletter: Subscription Form:
1. Go to Communication > Mail Specific Features > eNewsletter: Subscription Form
2. Select option 5: Form Customization
3. Click Add-Ons link on the top
4. Scroll down and mark the eNewsletter you want available on the form for sign up
5. Click Save & Done