धन्यवादः is Sanskrit. It becomes धन्यवाद ('dhanyavād') in Hindi
In Hindi, ः /'ḥ' (called 'visarg') is pretty rare. It is only retained in a handful of words and dropped from most other words like धन्यवाद.
Also, धन्यवाद is not literally 'thank you' so it doesn't change form depending on the form of 'you' used. The literal meaning is something like 'saying that one is fortunate/thankful'.