1) set up your 2 properties schemaName and tableName .. you can do this at this test case level, the testsuite level or the project level. (Highlight the testCase/testSuite / project in the left panel. If the properties window pane does not display below this panel, click View / Properties from the menu. Click on the "Custom Properties" tab in the properties window pane. Click the green plus sign to add the property, give it a name and a value.)
2) once the properties are set up ... go back to the SQL Query window in your screen shot and right click. This should bring up the 'Get Data' window. Select the properties that you want to use in your SQL query.
