What is the difference between send something somewhere and send something in somewhere? For example:
You should defintely send the tape to a music company.
You should defintely send the tape in to a music company.
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For what it's worth, "send in" carries a connotation of "submitting," like "submitting paperwork" or "submitting an entry for a contest." In the context of "Back to the Future," this would make sense, since Jennifer is suggesting that Marty submit his tape to a music company for them to listen and possibly offer him an opportunity. Omitting "in" here would be perfectly fine as well, but would not carry this connotation of "submitting."
You wouldn't use "send in" for other situations like
You should send the letter
into your grandmother.