What exact information needs to be included in a supplemental claim with new evidence to the VA?
The new evidence that is required is classed as information that was not already submitted to the VA when they made a decision. Information is deemed to be relevant evidence if it proves or disproves something that is contained in your case
Can I use the tool if I have a My HealtheVet premium account?
Yes, a Premium My HealtheVet account will do just fine.