Hi Joshua,
This code is not for dropdown list,This is for value help.
'WDR_TEST_DATE' Is nothing but the search help for date,so just create your own search help and give that name in the iv_help_id.
I know seach help will automatically comes when you declare variable as DATS type.
but you want to trigger server round trip for that reason i have given you this solution,
Hope it will solve your problem definately
Thanks & regards
NIkhil H