Adding PGP key for your repo Debian/Ubuntu

gpg --keyserver pgpkeys.mit.edu --recv-key insert_key_here
gpg -a --export insert_key_here | sudo apt-key add -

source: https://help.ubuntu.com/community/SecureApt

UPDATE:
another solution:

#!/bin/bash

# samu 2010-01-01
# (c) 2010 Samuele ~redShadow~ Santi
# redshadow&hackzine.org - http://www.hackzine.org
# Under Gnu GPL v3

KEYID="$1"

echo "Registering APT key $1..."
gpg --keyserver pgp.mit.edu --recv-keys "$KEYID"
gpg --export --armor "$KEYID" | apt-key add -

Source: https://wiki.hackzine.org/sysadmin/debian-register-apt-repo-key.html

ANOTHER UPDATE:
above didnt work with error message:

gpg: keyserver receive failed: No keyserver available

gpg: WARNING: nothing exported

gpg: no valid OpenPGP data found.

so another try:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 04EE7237B7D453EC

which eventually worked

Not sure what happened as the first command used to work for me in the past.