There are lots of reasons to attach an actual file, even if you think your question is simple. The code you posted is not sufficient to give you a reliable answer to your question.
Note that you are using an Input Box, not a Message Box.
There is nothing in the code you posted that would cause the checkbox to be unticked. Not sure why you expect that to happen.
Where is Line declared and what is its value when this code runs?
What kind of response is the user expect to give in the InputBox?
Based only on the one line of code you gave us I might try this.
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