Hey Ben,
First, you will need to setup the Recaptcha custom field. To do so, follow the instructions below:
1. Head over to Applications > Custom Fields
2. Search for "recaptcha"
3. Click on the app.
4. Insert the public and private key that you obtained from
https://www.google.com/recaptcha/
5. Save the custom field.
6. Insert the custom field into your profile workflow