I believe there are two steps for this...
First, on the tvheadend webUI - go to the CONFIGURATION > CHANNEL/EPG > EPG GRABBER MODULES tab and enable the External: XMLTV grabber.
Then in Kodi, in the Tvheadend service addon settings, go to the XMLTV category and set the XMLTV source type to WEB and put your web link address in the XMLTV Web location setting.
Then you can either trigger the internal epg grabber on the tvheadend webUI to update or reboot and see if it goes and gets the data. (do note that if you reboot, it may take a couple of minutes before the grabber runs)
Hope that helps.