connection String and Listener I am using SQL server 2017, I will like to know how Connection String can be use to talk via Listener to Databases rather talking directly to the DB
Availability Group Listener has a DNS name, for example AG-Listener-01.domain.com

In the connection string, you just specify 

Server=AG-Listener-01,1433;Initial Catalog=YourDatabase;User ID=SQLLogin;Password=SQLPassword;
MultiSubnetFailover=True