Launch a terminal for the desired TemplateVM then enable the RPM Fusion repositories:
sudo bash dnf config-manager --set-enabled rpmfusion-free rpmfusion-nonfree dnf upgrade --refresh
Now install telegram-desktop
dnf install telegram-desktop

It should be noted that if you enable RPM Fusion repos and use KDE packages the version of Qt or KDE provided by Fusion will likely break virtually every KDE package except konsole and updating Qt amicably will eject Telegram. For this reason you should consider duplicating your Fedora TemplateVM before installing Telegram or using the RPM Fusion repos in general.

Don't forget to reduce your storage limits in the settings.