Hi, Susan,
Here are two ways:
(1) Enter the symbol into the body of the document, using the AutoCorrect
entries (tm) or (r), from the Insert > Symbol dialog, or by pressing
Ctrl+Alt+T for the T symbol or Ctrl+Alt+R for the ® symbol. Then copy or cut
it to the clipboard, and paste it into the field.
(2) In the field, hold down the Alt key while you type a four-digit number
on the numeric keypad (with NumLock turned on): 0153 for the T symbol, or
0174 for the ® symbol.