what you create in C2 is HTML and JavaScript.
I doubt that you will get the answer here that you are looking for, as everyone here uses C2 and not GM. Easiest to get an answer on that is by doing a screenshot of how the textbox looks like, head over to the GM Forum and ask there how it is possible to do it in GM.
And if you want to know how it works, go to the plugin folder of your C2 installation and look at the code for the textbox (which is JavaScript and HTML).