Categories are stored in array field and you should implement a way to convert it to string. For example concatenating the values with ",".
To achieve this, you need to override ContentInboundPipe's SetProperties method and replace the default ContentInboundPipe registration with yours
In SetProperties method just call the base method and add your categories property as string. Something like "wrapperObject.SetOrAddProperty("MyCustomCategoryField", "mytag1, mytag2, ...");".
After that, you can just use this field as a normal search field like described below.
Please contact me again if you have any problems with this.
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