Jolena's answer in the comment section is completely wrong and I have never seen a person from Guangdong who speaks such terrible Mandarin.
The other answers are very good. I would personally say "先谢了" on casual occasions.
2019年10月29日
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A good question!
先 :in advance
多谢: Thank you so much
你: you
Mandarin style: 先多谢你!
*For extra learning:
Cantonese style : 多谢你先!
Cantonese usually put 先 at the end of a sentence.