Hello .,
I saw your question and thought that it would make a good feature request so I did some digging and found that this has been requested (UETOOL-86) and that it has been sent to the developers for further consideration. I went ahead and bumped up the community interest for this issue as well. I hope that this information helps.
Edit: Added workaround
Here I deleted the canvas panel and made the text block a variable.
I created a new OnGeneratedWidget event for the Combo box.
I then used the event to create a widget with the custom text in it.
Make it a great day