The platform for making declarations of travel in road transport is not IMI, but the following portal, which can be accessed through this address: https://www.postingdeclaration.eu/landing. The IMI system can only be used by the competent authorities.
In the following link you can find answers to the most frequent doubts that may arise (accessible in Spanish by changing the language above to the right): https://www.postingdeclaration.eu/help