Connecting to an Appian API

Certified Senior Developer

I've made an API in Appian, but am having the hardest time understanding how exactly one connects to it externally. 

According to the documentation: https://docs.appian.com/suite/help/21.4/Web_API_Authentication.html

I should be able to connect to it this way: example.appian.com/.../endpoint -H "Appian-API-Key: ${API_KEY}"

I put in my endpoint and API key, but every time I get an authentication error, so clearly I must be doing something wrong. 

Could someone walk me through the exact steps to get this to work? 

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