Must it be an X? Would a checkbox suffice?

AFAIK, you can't do what you're trying to do. You could put two labels, one
on each side of the text box with ( in one, and ) in the other.

So you just want the (   ) to show on the form, not be a part of the
value actually entered.

Add a label to the form. Size it wider than the text control.
Size it's height to the same as the text control.
As it's caption type
(       )
Use as many spaces between the () as needed to show the parenthesis
around the text control.
Make the label Back Style Transparent.
Move this label on top of the text control.
While the label is still selected, click on format + Send To Back.

You may have to resize the label or add or delete some spaces between
the parenthesis. Also, you may wish to make the font size of the
parenthesis a bit larger. That's up to you.
Once done, the view will look like ( X ).
Since the label is behind the text control, you can enter a value in
the text control.