It can be confusing to know when and when not to use a hyphen with thank you.
Thank-you is the noun form:
— I sent my brother a thank-you.
— Be sure to follow up with a thank-you.
Thank-you is also the adjective form
— She sent the teacher a thank-you note.
— A thank-you gift will be welcomed.
In verb form, use thank you.
— I thank you for your kindness
— Thank you for being there for me.