The answer as to whether you should or should not hyphenate 'thank you' really does depend on how it is used.
As a VERB - Do not hyphenate.
- Thank you for meeting me.
- I thank you for this opportunity.
As an ADJECTIVE - Hyphenate.
When 'thank you' is used as an adjective before a noun, it must be hyphenated. We do this to show that the adjective acts as a single idea.
- Holly likes to send thank-you cards for presents she receives.
As a NOUN - The grammatically correct answer is to either hyphenate, or to create one word.
I'm going to allow you to use your own judgement on this one. My personal preference is to avoid using
'thank you' as a noun. I would use a word like 'gratitude' if the context is formal, or 'thanks' if the context is informal.
- I would like to offer my gratitude for services rendered. (formal)
- Giving thanks before dinner is a family tradition. (informal)