Here's a screenshot of how to get the API Key. Make sure you write it down.
How to get the variables of your ManyChat user?
If you take a look at the JSON at the bottom of this page, you'll notice the first few variables contain values between brackets (). These have to be replaced inside your ManyChat member area. You can get the custom fields in the drop down menu in the dynamic content section where it says Add a Variable.
Copy the Code below this line and paste it inside ManyChat's Dynamic Response section
The values between () need to be replaced in your ManyChat section!