I think you mean: Enter a data into a field, (not add field to form), correct?
If so, the field on the form would be locked. Add a button to 'edit'
the user enters password, the field unlocks.
Code:
sub btnEdit_click()
txtField.locked = IsEditPassword()
end sub
sub function IsEditPassword() as Boolean
dim vRetval
vRetval = inputbox("Enter Password","Edit field")
select case vRetval
case "MyPass"
IsEditPassword = false 'we want the opposite to unlock the field.locked = false
case else
IsEditPassword = true
end select
end function