Entered Chat string variable

I was wondering which variable can be put in:

Event: Player types a chat message <variable> as an exact match.

i have tried 2 types of string variables, one undefined that i set in a trigger which doesn't work, and one with a predefined initial value and still no option to choose a variable :[.
i wish to use it many times or change it without having to manually make a new trigger, if that makes any sense. :\

A chat string trigger event can only be registered with a static string, you can not change the string on the fly, the only way to change it is to destroy the old trigger and create a new one with a new event.

If you want a more dynamic chat trigger then your best bet is to make it run for all chat messages (by making it respond to every chat message that contains an empty string as a substring) and then check what the chat message was with an if statement inside the trigger's actions. There, you can compare the chat message to dynamic variables rather than just static strings.