Keyboard Shortcut in "Specify Calculation" Window

[Esme]

>>using Windows XP

Does anyone know of a keyboard shortcut to write a field reference into the entry text box? I can select the field by tabbing to the fields, arrowing to my selection and pressing the spacebar to highlight it, but when I hit , it simply closes the window. I've tried any combination of and and , but nothing seems to work.

Anyone know the magic?

Thanks,

Esme