You can try a singleValueLegacyExtendedProperty for outlook to sent custom key.

create key when send

 const replyMessage = {
            comment: comment
            message: {
                singleValueExtendedProperties: [
                    {
                        "id": "String {guid} Name Property-name",
                        "value": "String"
                    }
                ],
            }
        };

example guid: 66f5a359-4659-4830-9070-00040ec6ac6e

and on the event side you can fetch with expand.

const message = await this.graphClient
                .api(`/me/messages/${messageId}`)
                .expand("singleValueExtendedProperties($filter=id eq 'String {guid} Name X-CRM-IGNORE')")
                .get();

Hey here is the resources.

https://learn.microsoft.com/en-us/graph/api/singlevaluelegacyextendedproperty-post-singlevalueextendedproperties?view=graph-rest-1.0&tabs=javascript