Congratulate sb on (doing) sth: ‘The President congratulates the crew on the success of their mission.’ ‘They congratulate him on his marriage.’
‘Congratulations’ is a plural noun: ‘Many congratulations to you both. I’m sure you’ll be very happy together.’ ‘Convey our congratulations to them and say that we’ll be getting in touch soon.’
Congratulations on (doing) sth (NOT for): ‘Congratulations on your promotion. You certainly deserve it.’