How can I use 1000000 baud rate on Run on Target Hardware - Simulink?

Hi, I'm trying to work with this feature of the Simulink (Run on Target Hardware). I have installed the Support Package for Arduino Hardware and I want to use it with an Arduino Mega 2560, but when I select 1000000 baud rate in serial port 3 and apply it, it changes to 500000 all the time. I have tried with the other ports but it is the same.

I don't know if any configuration doesn't let me fix that value or it is not avaliable even though it appears.

Thank you.