How To Make ONE Segment In A Descriptive Flexfield To Be Read-Only?

I have searched this here in Oracle support and found the following:

How To Make A Segment In A Descriptive Flexfield To Be Read-Only? [ID 357858.1]

I tried to follow the following solution listed in my CUSTOM.PLL:

A) In the Developer's Guide, chapter 14, there are some APIs that can be called from a custom
pll, like FND_DESCR_FLEX.UPDATE_DEFINITION.

The DFF I am trying to restrict is called "PO Lines" with a context code 105 that has the ATTRIBUTE9 defined in one of the segments. I can't still make it work. What is wrong with the following?

if form_name = 'POXPOEPO' then