Inventory item modifying a custom profile field

[Ferrel]

Okay, so the attempted code for the item I am looking at:

PHP Code:

{$lang->new_level}: <input type="text" name="newlevel" value= {"$post['fid5'] = 'Level: 02';" } />

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Fix5 represents the custom field I want to modify
The trick is, I'm not sure what file to define new_Level in or if that is even necessary. Each item would write a static bit of text. It wouldn't ever change unless they then bought a subsequent item.