Use the bolded apt-key command to add the GPG public key.

W: GPG error: stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1F3045A5DF7587C3
W: The repository ' stable InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
root@computer:~# apt-key adv --keyserver --recv-keys 1F3045A5DF7587C3
Executing: /tmp/tmp.UA1ZzOZ6hE/ --keyserver
gpg: requesting key DF7587C3 from hkp server
gpg: key DF7587C3: public key "Skype Linux Client Repository " imported
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)

Edit 20180207

Same problem, different program, Oracle VirtualBox:

$ sudo apt-key adv --keyserver --recv-keys A2F683C52980AECF