Okay, so just a couple of clarifications.
I'm not sure what you mean by "initialize the datasource". Do you mean load it? Because the loadDataSource() doesn't appear as an option inside of the OnVariableInitialization event of the Application.
Am I to assume that the dropdown into which I am to insert the code is the dropdown that actually loads the chart datasource, i.e. the one that refreshes the page?
On that assumption, I inserted the following into the OnSelect event:
var value = DROPDOWN_COMPLEX.getSelectedValue();
DS_DUMMY_COMPLEX.setVariableValueExt("psEnter Complex Code", value);
DROPDOWN_COMPLEX.setSelectedValue(DS_DUMMY_COMPLEX.getMembers("OBJ_103", 100);
This is your code with my names. The last line generates an error: Cannot convert from "MemberArray" to "ListValue"