[PXSelector(typeof(
 //What table to look at (CRCase), and what related field you want to save into the DB (caseID).  Also a Where to filter for only status = open
        Search<CRCase.caseID
        , Where<CRCase.status, Equal<CRCaseStatusesAttribute.Open>>>
    )
 //The fields shown in the Grid.  You can add more or less, just keep within the { }s.  
 //To show fields from other tables, you need to add a join into your Search.
    , new Type[] { typeof(CRCase.caseCD), typeof(CRCase.status), typeof(CRCase.subject) }
 //The surrogate key.  What you'll see as the "Value" in the field
    , SubstituteKey = typeof(CRCase.caseCD)
 //Comes after the "-" (EX: customers will have "BAcctCD - AcctName" -> in this case BAccount.acctName is the DescriptionField)
    , DescriptionField = typeof(CRCase.subject)
 //Caches the results
    , CacheGlobal = true)]

Add this attribute to your field and you should be good to go.

I'd recommend asking this on StackOverflow. You'll get better help there on customizations:

https://stackoverflow.com/questions/tagged/acumatica