Of note, documentation at Home Assistant not clear in stating that you have to copy your validator key from Meraki Dashboard and add it with the other information shown to the configuartion.yaml on the HA documentation page Home Assistant Meraki
Create a random secret letters and number and copy that 'secret' back to secret in the post Url on the Meraki dashboard.
as for the URL it has to be you External Home Assistant URL whether duckdns or whatever method you use with added /api/meraki at the end. You may have to add the :8123 port depending on your setup.
Click Validate in your Meraki Dashboard and all should work...
Remember to restart HA after updating your configuartion.yaml