After trying a few times I got it to work! I have no idea why it didn't work the first 2 times I tried it but now it does....
In /etc/dnsmasq.d/03-custom-DHCP.conf I added the following code:
dhcp-ignore=tag:!known
dhcp-host=11:22:33:44:55:66,set:known
dhcp-host=11:22:33:44:55:66,set:known
EDIT:
Looking back I think that I broke the configuration the first few times by restarting Dnsmasq.service manually through systemctl restart.
The correct way is pihole restartdns after a config change