How do I add a range to a QueryBuildDataSource in X++?

Add a QueryBuildRange to the datasource and set its value.

QueryBuildRange range;

range = qbds.addRange(fieldNum(CustTable, AccountNum));
range.value(queryValue('US-001'));

For normal single values, use queryValue() or SysQuery::value() so the value is formatted correctly for a query range.