Gets and sets the property ExternalId.

A unique identifier that is used by third parties to assume a role in their customers' accounts. For each role that the third party can assume, they should instruct their customers to create a role with the external ID that the third party generated. Each time the third party assumes the role, they must pass the customer's external ID. The external ID is useful in order to help third parties bind a role to the customer who created it. For more information about the external ID, see About the External ID in Using Temporary Security Credentials.

public string ExternalId { get; set; }