Disabling Form Fields Server Side

Hello, How do I navigate, from an item to its associated form and then manipulate the associated controls. I want to disable a drop down based upon a field condition in the same form. I'd like to do this as a server side event (onAfterLock) I am able to achieve something similar from a client side as a form onLoad event but the fields do not stay locked once you've toggled the lock status. Appreciate any feedback.
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