My initial thought was that this wasn't possible, but hey it's Friday afternoon where I am so I decided to see if I could figure this out. I think I may have something that will work for you.
My example uses the eFashion universe.
The first step is to create a variable with no Formula to accept your filter value. I called mine "Store Name Filter Value"...
I then created a new input control based on Store Name Filter Value...
Select "Entry field" as the Control Type with a default value of an asterisk ("*") and set the operator to "Equal to".
Next I created another variable called "Is Match" that takes the value of "Store Name Filter Value" and checks if it matches that of "Store Name" which is a dimension directly from the universe.
Finally, I added a report filter on "Is Match".
Here is the result. You see all rows with the default value of "*" in the input control...
Now if I set my input control value to "*New*" I only the rows with the word "New" somewhere in the Store name.
As it is this is case sensitive, but you could make it case insensitive by using the Upper function within formula in the Is Match variable.
Enjoy,
Noel