To create an external API user, open the Settings module and go to Users. Choose the drop down arrow and select External API User
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This brings up the External API User details. Enter in the details for the user including an identifiable name and any roles they need to have access to.
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Choose Save to create the user. Once created choose More Options and select Create external API session
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This generates a both the User and Session ID numbers for use with the API.
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Generating a new session will create a new session ID, resulting in previous sessions ceasing to work.