I'm not sure I understand the question. TestComplete, when using the "Keys" method, you will see on screen the keystrokes being entered in. So, I'm not sure what pop-up window would need to be displayed?
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Why automate? I do automated testing because there's only so much a human being can do and remain healthy. Sleep is a requirement. So, while people sleep, automation that I create does what I've described above in order to make sure that nothing gets past the final defense of the testing group. I love good food, good books, good friends, and good fun.
for key in keys:
And invoke it when required. You will see all inputs near the TestComplete icon. May be you will need to set a Key pressing delay (in Project Settings) to some significant value to make it visible on the screen.