how to include a simple license into an indicator

It depends on what type of licence you want e.g.

A time limit (indicator expires after a certain date)

A trading limit (indicator restricted to demo accounts only... more relevant for EAs)

A password check (indicator requires the user to enter a password to use it)

An account number (indicator can only be applied to specific accounts)

And so forth. 

The methods vary from the very simple to the fairly complex, depending on what it is you're actually after. 

The password check would be same password for all copies? Would make it worthless if someone repost the indicator together with the password.

Account number: would it need to recompile each time a client ask for a copy?

What is easiest solution for a sell here with this mql4 website?