You should first enable your extension field to be used in search scenario:
After that it should be available in Search UI:
Click hyperlink to make it visible in search criteria:
And now it works in UI:
Search Odata request:
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SAP S/4HANA Material Fiori应用根据扩展字段搜索的实现原理
原文：https://www.cnblogs.com/sap-jerry/p/12232819.html