This is a follow up to my question: SSIS connection parameter based on parameter/variable
I now can see how to construct a variable from other variables, but it does not appear possible to then use that variable as the ConnectionString for the Connection. That appears to only allow variables.
How can I use my newly concatenated variable as the ConnectionString?
Ok, I can answer my own question now. It is a different location than if you want to use a parameter to set the ConnectionString.
Click on the Connection in Connect Manager and in the Properties window, expand Expressions. There you can click the ... to set it to a variable.