On my site, users can add a link to a website. The validation checks if it has a scheme (e.g. http, ftp). If it has no scheme, it adds 'http'. But what if the user enters an ftp link, for example? I'm not sure how to explain to the user why their link is invalid.
I assume that a user who thought an ftp link was a website link has their hands on a few links associated with the project and just needs to identify the right one. I could tell them that they need to enter a link that starts with 'http', but they might not have been given the link with 'http' written in front of it.
So how can I help the user identify which link they should be inputting?