How to hide a text input box on a datacard in EPDM using control logic

I am trying to do something which I hoped would be simple but doesn't seem to work.  I have a part filecard which is the same for all of my parts.  I have an input field (dropdown) which also me to select part type.  When I select part type I want one of my variable input fields (PL Number) to be hidden.  Basically this is a manual input field which should only be visible for one part type.  I cannot get this to work!

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