Great question — answering as the UphillTech team:
When you create API Key(API_KEY_SID) - you'll be shown the Key's secret(API_KEY_SECRET), You'll use the the secret(API_KEY_SECRET) of the API Key(API_KEY_SID) you created in step 1 to generate an access-token(ACCESS_TOKEN) using the Twilio Helper Library Detailed Explanation here - Twilio Authorization - Refer the step 1,2,3, Its explained with example in different languages, including Nodejs.
Full answer by Srikanth Balaji: https://stackoverflow.stackexchange.com/a/41754420 (CC BY-SA 3.0).
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