I am not really sure why you want to sniff connection string when you already know which server it is connecting to, on which port communication is happening. Possibly, you are not sure about the login id and password. You may check with your DBA and he/she would help you with the login id. In case, you are not sure about the password, you may get it reset if you are the owner of this application or you are the only user. If above option is not favorable, in case somebody else is using that account, you may get another login instead.
Typically connection string contains three parts - Server Name with instance name(if any), login name, password, port. if port is default(as mentioned), you don't need port in the connection string.
Please refer below links for more understanding:
I hope above helps.