a question people I've seen these "Thanks in advance" and "Thanks beforehand" many times around.
My doubt is "are those correct or are those appropriate to use?".

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'Thanks in advance' is sometimes used, usually via email rather than verbally. I've never heard the phrase 'Thanks beforehand' being used, and it isn't a phrase I would use.

Hope that helps!

Answer

In a letter when you are asking someone to do something (quite formal) you may finish with "Thanking you in anticipation"
"Thanks in advance" would be informal, but I've not really seen it used much.  I have not seen or heard anyone use "Thanks beforehand"
Hope this helps