Thank you for your help!
Based on runtime error, the format is inst.addrepo=name,url
I used this command:
sudo virt-install -l
http://hkg.mirror.rackspace.com/fedora/releases/34/Everything/x86_64/os/ --disk
path=/dev/disk/by-id/ata-Samsung_SSD_850_EVO_250GB_S21MNSAFC02148L-part4 --name
f34-uefi-dpt --ram 2048 --boot uefi
--extra-args="inst.addrepo=rackspace,http://hkg.mirror.rackspace.com/fedora/updates/34/Everything/x86_64/"
It is exactly what I wanted as updates are installed.
Just that the option of "not install update" is greyed out in this way.