Paperback 
Install Zabbix Agent (Active) on a Windows Host Behind a Firewall
Video Lecture
Description
I download and Install the Zabbix agent for windows option.
I set the agents ServerActive parameter, and configure a template in Zabbix with Agent(Active) items only.
Since the template is using Agent(Active) items only, I do not need to create any firewall port forwarding rules on the host running the agent.
I do however, need to configure my Zabbix servers own firewall, to allow my remote host to send messages to my Zabbix servers port 10051.
Troubleshooting
Unable to connect to [Your Zabbix Server IP/Domain]:10051 [cannot connect to [[Your Zabbix Server IP/Domain]:10051]: (null)]
When a Zabbix agent starts up, it doesn't yet know the list of items it has been configured to check. So it will attempt to connect to the address listed in its ServerActive= setting in the zabbix_agentd.conf file.
If your agent is not configured to do an Active checks, then you can ignore this error.
no active checks on server [Your Zabbix Server IP/Domain:10051]: host [Your Host running Zabbix Agent] not found
Make sure you have set up your host using the Zabbix Server UI.
Whatever value you enter into the Hostname= option in the zabbix_agentd.conf needs to be the same value entered into the Host name field in the Zabbix Server UI → Create Host/New Host screen. The values are case-sensitive and must match.
You should also assign a template containing active items.
If you are not using any active items, then you can ignore this error message.