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System users represent servers or software that make API calls to assets owned or managed by a Business Manager
to add a system user to your Business Manager, follow this guide Add System Users to Your Business Manager
Go to Business Settings
Below Users, click System Users.
Click Add New System User
Give a name to the system user, give him Admin access and click Create System User
Assign your app as Assets to the user created and give the user full control
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Generate Permanent token