"Give me an address @HelloFreshUK, I'll send it to you and you can have a whiff in case you're sceptical," he then added.
The man said the box arrived unsealed and the liquid inside the Coke bottle was definitely human urine and not apple juice.
"No, it's not a joke! Boy do I wish it was," he said.
Since HelloFresh does not deliver drinks with its meals, some Twitter users speculate that the urine could have belonged to the delivery driver and been added to the man's box by accident.
After McManus' tweet went viral, the company responded to him on social media.
"We truly lack the words to describe how sorry we are because of this," HelloFresh told the man.
In a statement provided to UK media, the company said they were "horrified" that the customer had received the item.
"Given the wholly unacceptable nature, we are urgently investigating, along with our delivery company, how an open box with this bottle in it was delivered," the statement added.
"We have apologised directly to Mr McManus and offered an appropriate goodwill gesture.
"We are working hard to do everything we can to ensure that this cannot happen again."