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You need the following information to link your account: authentication key, GAME ID, Placement ID.
Authentication key: In Unity Dashboard choose Operate and then Ad Data Export. Click on API Access. In Statistics API field you can find your authentication key.
GAME ID: In your Dashboard go to Operate and choose Projects, click on the application name. Choose Monetization. You can find your GAME ID in the Platforms tab.
Placement ID: Go to Operate and click on Projects. Choose the app and in Monetization menu click on Placements. Select an ad format, click on it and on the next page you’ll see the Placement ID in General Settings.