Set Who can use this application or access this API? to Accounts in any organizational directory (Any Azure AD directory - Multitenant)
Set the Web redirect URI to <Cal.com URL>/api/integrations/office365calendar/callback replacing Cal.com URL with the URI at which your application runs.
Use Application (client) ID as the MS_GRAPH_CLIENT_ID attribute value in .env
Click Certificates & secrets create a new client secret and use the value as the MS_GRAPH_CLIENT_SECRET attribute