You are not being very clear here.
Do you mean that you would like the asterisk symbol to be displayed when someone enters a value in the field
OR
Do you mean that you would like to actually use the asterisk symbol as the password?
The former is set via the
TextBox.InputMask Property by setting the property to "password" - please follow the link for details.
The latter will require more detail about what you are actually attempting to accomplish.