Currently, pressing the enter key while typing in a string in the String tab of the String Editor (accessible via Quest->Strings->Edit/Add New Here) appears to do nothing. It would be more useful if ENTER inserted a newline or was an additional OK.
Make enter key do something in the String Editor
In the meantime, the questmaker can insert a 'Line Break' SCC if they want a newline.
See string_editor.cpp