OpenGameArt.org
Heroes of Might and Casual
Credit line
"Heroes of Might and Casual" by Alexandr Zhelanov licensed CC-BY 4.0: https://opengameart.org/content/heroes-of-might-and-casual Author's attribution notice: https://www.youtube.com/c/AlexandrZhelanovsMusic
Where to put it: Game credits or video description. If the entry has a "Copyright/Attribution Notice", follow that.