Custom lookup on Form in D365

 [Form]

public class MKCustomLookupForm extends FormRun

{

    [DataSource]

    class MKCustomDSTable

    {

        [DataField]

        class FieldIdCust 

        {

            /// <summary>

            /// </summary>

            /// <param name = "_formControl">Form control</param>

            /// <param name = "_filterStr">Fileter condition, if any</param>

            public void lookup(FormControl _formControl, str _filterStr)

            {

                CustTable   custTable;

                 SysTableLookup sysTableLookup = SysTableLookup::newParameters(tablenum(MKTable), _formControl);

        Query query = new Query();

        QueryBuildDataSource queryBuildDataSource;

        queryBuildDataSource = query.addDataSource(tablenum(MKTable));

        queryBuildDataSource.addSortField(fieldNum(MKTable, FieldId), SortOrder::Ascending);

        sysTableLookup.addLookupfield(fieldnum(MKTable, FieldId), true);

        sysTableLookup.addLookupfield(fieldnum(MKTable, FieldName));

        sysTableLookup.parmQuery(query);

        //logic

        sysTableLookup.parmTmpBuffer(fieldList);

        sysTableLookup.performFormLookup();

            }

        }

    }

}