You can find the unique BCC address under your user name in the top right corner of Salesforce. From there, click Setup > My Settings.
Now, select the Email tab and My Email to Salesforce.
On this page, you will see the unique BCC address (your Email to Salesforce address)
Just copy and paste this address into the BCC address section of the Yesware > Preferences > Compose Options drop down menu in Gmail.