I know for a fact that having a link like this for example: <a href="sms://12345">send message to 12345</a> will open up the Messages app with a prepended number 12345 and a way to start writing a new message to that number.
I can’t seem to figure out how to just open the list of all the messages, without sending in the number. If I do it like this: <a href="sms://">open Messages</a> then nothing happens when you click the link.
It could be that this is not at all possible, but then again, I'd like someone to confirm that please.