If you are a user looking to impersonate someone else, please see Impersonating
Before users can impersonate someone else, it must be enabled for each app.
To do this navigate to Apps.
Then choose the app for which you wish to enable impersonation.
Navigate to Access Policy, check Enable Impersonation, and press Save.
If you are an administrator, you may pre-authorize impersonation. This isn’t strictly required, because users can also request impersonation as needed.
To pre-authorize impersonation, navigate to the Impersonation tab for an app and click Add.
Then select the user who is doing the impersonating on the left, and the user who is being impersonated on the right.
You will now see your user listed in the approved impersonation pairs.