This isn't possible in Git Extensions. The commit panel maintains its state and cannot default to any value. As an example, open the commit window and type "This is my commit text" or some arbitrary message into the commit area, then close the window and the entire application without doing the commit. After you restart the application, the text will still be in the commit area. This can be pretty useful, but as a side effect of maintaining state, it appears we lose out on any sort of defaults besides a blank message.
The only workaround I know of is clone the Git Extension code repo and add the feature yourself and request that it be included in future versions. Might be more work than it's worth for you.