Sorry, I thought the option was available in YnABMC when exporting, it's not, so add this in the <Parameters> section of your map's confix.xml, changing "YourMapName" to yours:
Code:
<Replace Key1="Map" Key2="YourMapName.lua" ParameterId="FloodPlainsPlacement" Name="LOC_MAP_FLOODPLAINS_PLACEMENT_NAME" Description="LOC_MAP_FLOODPLAINS_PLACEMENT_DESCRIPTION" Domain="FloodPlainsPlacement" DefaultValue="PLACEMENT_DEFAULT" ConfigurationGroup="Map" ConfigurationId="FloodPlainsPlacement" GroupId="MapOptions" Hash="0" SortIndex="260"/>