I made my own preamble and saved it into a separate file. I include it with
Now in the preamble there is
Now I want to pass MyTitle from my main document. How do I do that?
You could take that line out of
Preamble.tex and put it in the main document. Which is more useful, if you want to share the preamble code with other documents.
Incidentally it would be more idiomatic latex if you called your file
preamble.sty rather than
Preamble.tex then you could use