Why would you do this? For example: "Any transformation or change in the original characteristics of the vehicle must be communicated to the IMT", so it's good to get yourself familiarised and registered, which you can do using your NIF/Financas Portal details at the address above!
PS I also just paid a 'portgens' toll road bill at the local post office, after prompts from this handy system (this for anyone who hasn't opted for the Via Verde in-vehicle transponder system). Have you registered with their online services? Feel free to add your comments and insight.