Enable PSK in Zabbix

PSK means pre shared key.
The PSK option consists of two important values, the PSK identity and the PSK Secret.
On the agent - generate a PSK secret:

openssl rand -hex 32 > /opt/secret.psk
chown zabbix:zabbix /opt/secret.psk
chmod 640 /opt/secret.psk

Reconfigure the Zabbix agent by opening - /etc/zabbix/zabbix_agentd.conf:

TLSConnect=psk
TLSAccept=psk
TLSPSKFile=/opt/secret.psk
TLSPSKIdentity=[any text]

Next:

systemctl restart zabbix-agent

Then go into the Zabbix Server User interface and configure the PSK encryption for the host - select the:

'Connections to host' - PSK
'Connections from host' - PSK
'PSK Identity' - [what you used in the Zabbix agent config]
'PSK' - [use cat /opt/secret.psk and copy the value here]

After a minute or two, the Zabbix Server and Agent will successfully communicate using PSK encryption.