How to access full email source code in thunderbird message_display_action extension?

Question

I am building an extension in Thunderbird with UI element message_display_action for my school project. I am wondering if I can access full email source code from java script file that I am using for building up my html page when pressing on button my extension. I found a funciton named getFull(messageId) in documentation at https://webextension-api.thunderbird.net/ but I don't understand how to use that function and I don't even know what messageId is. I know it is a integer but I don't know how to get that integer for a specific email. I entered permission (messagesRead) in my manifest file of extension for reading emails but i still don't know how to use that function. I didn't find any examples or tutorials on the internet so if anyone can help me or atleast point me to the right direction.