Could someone explain how to create a cookie via irule?

It looks to me you are confusing the cookie Name with a cookie Value. Your pre-existing line of code checks if a cookie with a Name "targetCookie" has Value "" and if yes then send the traffic to the backend node (server) with specific IP address.

The line you are trying to wrong because the cookie name should be "targetCookie" and value should be the server name.

You should be putting all string parameters in double quotes.

HTTP::cookie command documentation is available here:

https://clouddocs.f5.com/api/irules/HTTP__cookie.html