It is defined in the Cross Currency Conventions. I don’t see an example of XML on GitHub, but you can see the code to parse the XML here
https://github.com/OpenSourceRisk/Engine/blob/93b8ed8778cf9799707151d552607e44d87bf130/OREData/ored/configuration/conventions.cpp#L677
So set the optional flag to True (The default is False) in your You can also specify which leg is resetting here (default is the flat leg)
Here is an example
<CrossCurrencyBasis>
<Id>EUR-USD-XCCY-BASIS-CONVENTIONS</Id>
<SettlementDays>2</SettlementDays>
<SettlementCalendar>TARGET,US,UK</SettlementCalendar>
<RollConvention>MF</RollConvention>
<FlatIndex>USD-LIBOR-3M</FlatIndex>
<SpreadIndex>EUR-EURIBOR-3M</SpreadIndex>
<IsResettable>Y</IsResettable>
<FlatIndexIsResettable>Y</FlatIndexIsResettable>
</CrossCurrencyBasis>
Regards,
Niall.