C# SQL Connection String seems to be forcing windows authentication

Server=LABLAPTOP-1\\SQLEXPRESS;database=MyDB;userid=sa;password=1234;Trusted_Connection=yes;connectiontimeout=30");

This works when I work locally on the SQL server, however if I put in the wrong password it still works so clearly it is using Windows Authentication.

When I use a PC in the same LAN (WORKGROUP - not domain) I get "the login is form an untrusted domain and cannot be used with Windows Authentication."

Help would be much appreciated!