How do i wait until a String returns a value?

How do i wait until a String returns a value?

i have a program that uses the KeyPress event to add a new character to a
new Label, sort of like a Console Application. I need to add a Input
method into my program so when i press enter instead of executing a
function, it returns a string. i have tried making the KeyPress event
return a string but it wont work for obvious reasons, how do i make this
work?
Note when I wont to return a string i mean if i where to ask the 'Console'
to wait for an input i would still be using the KeyPress event but it
would return a string/input from the user.