The US Federal Trade Commission puts it this way:

a firm with market power cannot act to maintain or acquire a dominant position by excluding competitors or preventing new entry

It further explains that “market power” means:

the long term ability to raise price or exclude competitors

Emphasis added. What the government might argue in this case is that Apple has market power in the online message space because it preloads its own messaging app on its smartphones, which I believe enjoy a majority market share in the USA. One remedy the government could seek is requiring Apple to allow third parties to develop clients for its messaging service.