// get the user option for default currency
var userOptions = Sage.Platform.Application.ApplicationContext.Current.Services.Get<Sage.Platform.Application.Services.IUserOptionsService>();
var defaultCurrency = !string.IsNullOrEmpty(userOptions.GetCommonOption("lveCurrency", "OpportunityDefaults"))
    ? userOptions.GetCommonOption("lveCurrency", "OpportunityDefaults")
    : "USD";

// now set for opportunity
opp.ExchangeRateCode = defaultCurrency;