Hi Mark - you can obtain this by logging in to your AMP Threat Grid [TG] cloud account [https://panacea.threatgrid.com]. The API key is available under your username -> My Account.
Note: Access to the AMP Threat Grid console requires a AMP Threat Grid subscription. If you have AMP for Networks, AMP for Endpoints, or some other integrated use of AMP Threat Grid [ESA/WSA/etc], those AMP Threat Grid integrations do not include a login to the Threat Grid console.
Screenshot attached.
Hope this helps,
ELH
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ELH