Although Flow does in fact have an API, it would not be practical to integrate with it at the present time.
Flow is used on a vanishingly small number of wikipedias (as the default discussion system), so there is very little chance of incompatibility with user talk pages on any wiki.
And even then, if we really wanted to integrate with Flow, it should be done at the level of the page/talk backend service, not at the client level.

In T261660#6433604, @Dbrant wrote:

Although Flow does in fact have an API, it would not be practical to integrate with it at the present time.
Flow is used on a vanishingly small number of wikipedias (as the default discussion system), so there is very little chance of incompatibility with user talk pages on any wiki.
And even then, if we really wanted to integrate with Flow, it should be done at the level of the page/talk backend service, not at the client level.

Thanks for investigating. Not something we will support natively at this time then - we can link to a web page in the few cases where this may be necessary.