I know you wanted only a hypertext link, but if you copy & paste a link address into Slack that does work very nicely. i.e. if referring to VersionOne ticket number (V1 mouseover the ticket window to open the mouseover window, then right click on the ticket number for the option to "copy link address", then in Slack paste. It'll paste the full ticket URL but then it shows a nice summary of the ticket number and name and you can click it to go right into the ticket.)