The below script is not recommended if you can install the keys from a keyserver (as recommended in another answer using apt-key adv) or if you can download them from a trusted source via https and install using apt-key (eg wget https://trusted.key.site/my-trusted-key.gpg | sudo apt-key add -), but if you don't have ANY other way, you can use this.

echo "deb http://your.repo.domain/repository/ $(lsb_release -c -s) universe" | sudo tee /etc/apt/sources.list.d/your-repo-name.list

sudo apt -o Acquire::AllowInsecureRepositories=true \
-o Acquire::AllowDowngradeToInsecureRepositories=true \
update

## if the 'apt update' above fails it is likely due to previously
## having the GPG key and repository on the system, you can clean
## out the old lists with `sudo rm /var/lib/apt/lists/your.repo.domain*`

apt-get -o APT::Get::AllowUnauthenticated=true install repo-keyring-pkgname

## If you ever run `sudo apt-key del your-repos-keyID`
## you may have to `sudo apt remove --purge repo-keyring-pkgname`
## Update should run without the GPG warnings now that the key is installed

apt-get update
apt-get install somepkg-from-repo
I originally put this together because i3 in their sur5r repo does this, but then I found out their keys are in the keyserver.ubuntu.com list, so I can just sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E3CA1A89941C42E6 and avoid all the extra package hassles.