Unable to make Master Contract Field as Mandatory based on Doc Style

Summary: Unable to make Master Contract Field as Mandatory based on Doc Style

Content (required): We have a requirement to make the Master Contract Field as mandatory in Purchase Order header based on the Purchase Order Style chosen. So please let us know if anybody knows the process. We have used below EL Expression but it doesn't help:

#{bindings.DocStyleName.inputValue=='Purchase Order - Tender'}

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