Although this isn't a general answer to the default action problem, in this program with one editable text field and a simple button action, adding the action takes two lines of code. This is possible because we already have the widgetDefaultSelected method declared in GraphicUI.java. No object is calling that now.
When the user presses return in the text field, that activates the default selection. So we need to have the text field add a selectionListener that will be implemented by the GraphicUI object (called this inside the object). Add that below the line for addModifyListener in createTextItems.
Then add the action you want to happen inside the widgetDefaultSelected method toward the bottom. I didn't send the message to the button. Instead, I just did what was done when the button was clicked and called findWord.
That was all I changed and it worked.