Well, I don't know how to technically do this, but what my experience currently with these two (WeChat and Line) is that if fill in the WeChat/Line ID when setting up the button and then click it on my site （with another people's phone who is not a freind of me on WeChat/Line）, I will be redirected to WeChat/Line Interface where you can see the chats list, not the chat window between the client and me. In real practice, take WeChat as an example, the client is new to my site and would need to add me (even if I have turned off 'Friend Confirmation' in WeChat) manually by copying the ID and paste it to the 'Add Contacts' field. This also require the ID to be included in the button name and memorised by the client. What I hope can be achieved with Buttonizer is that the ID will be prepopulated when the client click the button (and I don't have to include the ID in the button name as it is a bit long), so whenever the client clicks at, say, a button named 'Talk on WeChat'， he will be seeing the chat window with me and talk straight away without any other actions.
Similarly, a return visitor who is already a friend of me on WeChat, he will be redirected to the chat window after clicking this button and start typing his/her questions straight away.
I hope this makes sense. Thanks!