The usual way to do this is to create a small script that grabs the data for TVH.
The script can either write the data to a file, or pipe it into the TVH socket.
For a file, you'll need to enable INTERNAL:tv_grab_file, for the socket you'll need to enable
EXTERNAL: XMLTV - these can be found in Configuration Channel/EPG EPG Grabber Modules.
You may need to install tv_grab_file if you want to use it & your system doesn't have it already.
You script should do something like:
curl -sSL 'URL' > /tmp/xmltv.xml
or
curl -sSL 'URL' | nc -w 5 -U .hts/tvheadend/epggrab/xmltv.sock
Note that there are several versions of 'nc' and only one of them is capable of writing to a Unix socket,
you may need to use 'socat' instead you should be able to find examples by searching the forum.
Run your script manually while looking at the log file in the TVH gui [click on the icon that looks like two ^ at the bottom right],
if it works you will see a message indicating that episodes were updated.
Once you have the script working, use crontab to run it once per day.