As noted in the README

NOTE: This Action (currently) only supports auto-adding Issues to a Project which lives in the same organization as your target Repository.

and also raised on the feedback forums (without ack from GH) it is not possible to add issues to a board that belongs to a different org. As this is possible in the UI, it appears to be a validation issue in the GraphQL API.

My question / suggestion is, will this ever become possible? It would be a huge relief in managing multi-org set ups that popular open source ecosystems use, such as Ory.

Thanks! :)