To enable the ability to approve or reject changes through Gmail, you need to:
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For best practice information on securely using API keys see the Google Support documentation (https://support.google.com/googleapi/answer/6310037?hl=en&ref_topic=7013279).
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 Open https://console.cloud.google.com/apis/credentials. Review the Terms of Service and click Agree and Continue.  | 
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 Select a project and click New Project.   | 
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 Click Create Credentials | OAuth client ID.  | 
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 Click CONFIGURE CONSENT SCREEN.   | 
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 Select the user type and click CREATE.   | 
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 Enter the application name on the OAuth consent screen and click SAVE AND CONTINUE. Although some fields are optional, the following information is required:  | 
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 To add the GMAIL API from the library, select ADD OR REMOVE SCOPES, click Google API Library link, enter GMAIL API in the Filter field, choose Gmail API, and click Enable.  | 
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 Check the Read, compose, send, and permanently delete all your email from Gmail checkbox, then click UPDATE.  | 
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 Click SAVE AND CONTINUE to select your test users.  | 
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 Click ADD USERS.   | 
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 Enter the required account’s email and click ADD. (At a minimum, the account used to connect must be assigned as the test user.)  | 
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 Click SAVE AND CONTINUE to review your settings.  | 
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 Click BACK TO DASHBOARD.  | 
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 Click CREATE CREDENTIALS.  | 
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 Click Download to download the credential information in JSON format.   |