Now I understand.
The resulting code in the body tag looks like this:
With this code the size selection (child menu) will be set to size M, but only if I delete the behaviour that populates the size selection with the available sizes. Unfortunately showing the available sizes is more important than being able to preselect the most popular size M.
For future versions of DataBridge it would be nice, if the selection in the child menu can be set while at the same time it is possible to populated the child menu with the available options only. Also when the user makes a choice in the child menu first and then decides to change the selection in the parent menu, that the child menu is not reset, but stays at the previous user selection.
Sorry, for my English. I hope I was able to explain the problem.
And thank you, Jason, for helping me.