What I'm proposing is that all/most of the predefined sites should maybe have a placeholder="" on the input field for the url so that people know what kind of url to put there.
For most platforms this is obv but maybe for some federated services this might be more confusing.
Obv people don't have to follow this url guide, it would be just to help people.

A good example: Matrix. Most people don't know how their matrix account should be 'linked' to.
So displaying this might help:
image

<input type="url" name="link" value="" class="form-control" required="" placeholder="https://matrix.to/#/@USERNAME:SERVER">