Knowing how to be polite in French is essential for any French language learner. You can use the preposition “de”:
If what follows thank you is a French verb
When you are saying thanks for an intangible gift.
Examples
merci de me rappeler: thanks for reminding me (you have the construction: merci + de + verb)
merci de ton aide: thanks for your help (you have the following construction: merci + de + intangible help)