This is from help Doc. Hope this helps!!

-> On the View menu, click Field Explorer.
-> Select Parameter Fields and click New.
   The Create New Parameter dialog box appears.
-> Enter the Name and value Type.
-> For a string parameter field, you can choose to enter an edit mask in the Edit mask field, rather than specifying a range.
   An edit mask can be any of a set of masking characters used to restrict the values you can enter as parameter values (the edit mask also limits the values you can enter as default prompting values).
   You can enter any of the following masking characters, or any combination of them:
         "A" - (allows an alphanumeric character and requires the entry of a character in the parameter value).
         "a" -  (allows an alphanumeric character and does not require the entry of a character in the parameter
                     value).
         "0" -  (allows a digit [0 to 9] and requires the entry of a character in the parameter value).
         "9" -  (allows a digit or a space, and does not require the entry of a character in the parameter value).
         "#" -  (allows a digit, space, or plus/minus sign, and does not require the entry of a character in the
                     parameter value).
         "L" -  (allows a letter [A to Z], and requires the entry of a character in the parameter value).
         "?" -  (allows a letter, and does not require the entry of a character in the parameter value).
         "&" -  (allows any character or space, and requires the entry of a character in the parameter value).
         "C" -  (allows any character or space, and does not require the entry of a character in the parameter
                    value).
         ". , : ; - /" (separator characters). Inserting separator characters into an edit mask is something like hard coding the formatting for the parameter field. When the field is placed on the report, the separator character will appear in the field object frame, like this: LLLL/0000. This example depicts an edit mask that requires four letters followed by four numbers.
         "<" -  (causes subsequent characters to be converted to lowercase).
         ">" -  (causes subsequent characters to be converted to uppercase).
         "\" -  (causes the subsequent character to be displayed as a literal). For example, the edit mask "\A" would display a parameter value of "A." If the edit mask is "00\A00," then a valid parameter value would consist of two digits, the letter "A," and then two additional digits.
         "Password"-   Allows you to set the edit mask to "Password," you can create conditional formulas specifying that certain sections of the report become visible only when certain user passwords are entered.

Note: Some of the edit mask characters require that you enter a character in their place (when entering a parameter value), while others allow you to leave a space, if needed. For example, if the edit mask is 000099, you can enter a parameter value with four digits, five digits, or six digits, since the '9' edit mask character does not require the entry of a character. However, since '0' does require such an entry, you could not enter a parameter value with less than four digits.
-> Click OK.
-> Return to the Field Explorer, and drag the parameter into the report.