Add Privacy Checkbox To Contact Form & Email Optin

For both of these you will need to use the Visual Builder.
For the Contact Form:

1. Open it and click on Add New Field.

2. Set a title to it and select the Checkboxes type from Field Options. You'll see a checkbox added instantly to your form: 

3. In the Options field write your text with the link to the website's policies: 

Here's a live demo of the above: 

For the Email Optin: 

1. After you select the Service Provider (I've did this example with MailChimp) go to Email Optin Settings -> Fields and click on Add New Custom Field: 

   2. Add the custom field which is created in the Service Provider: 

Further customize that field and make it Required if needed and add links too: 

Here's a live demo of the above: