Personally I'd prefer the presets included a link to my mapshare page rather than just current location; because these messages are free (vs paid). I guess the workaround is to create a bit.ly link to my MapShare link and include that in the preset message text.
He assumption is that interested parties know the URL for your MapShare page. The target of the “location link” sent with the message is just a convenient way to see your location at the time the message was sent on a map, to see message details, and (email messages only) to reply.
There is a way to send a “share” message that includes a link to your MapShare page. The option is offered at least when you start tracking. This message is not free, however.
To include tracking information in your preset message, simply include your mapshare link in your preset message, for example:
"At the start. My track: share.garmin.com/<<5char_user_name>>. "
All of your preset contacts can then just click your mapshare link in the received preset message to track you. And of course, the sending of the preset message is free.