Prompting player to input a string at beginning of game

[swni]

It was suggested in a response to another of my questions that I can ask the user to input a string at the beginning of a game. However, I couldn't figure out how to do that. I assume this involves the LuaGui stuff in some way, which I've never used before, but after looking through the documentation I couldn't find how to proceed.

Ideally I would like to create a (preferably modal) dialog box at the beginning of the game that has some text, has a text box for the player to type in, and a button to confirm.

I could have players input the string via a command at the console but I believe this might be unfriendly to some players and I think it disables achievements.