How do I add additional customBodyField criteria in OpportunitySearch C#

@operator

I have a piece of code that runs the OpportunitySearch and now I need to add an additional parameter to the following search:

basic = new OpportunitySearchBasic
    {
              lastModifiedDate = new SearchDateField
        {
            @operator = SearchDateFieldOperator.after,
            searchValue = DateToStartSearchFrom,
            searchValueSpecified = true,
            operatorSpecified = true
        },
    } 
   
});

The additional parameter would be a boolean custom Transaction Body Field that I will need to check for true value. It looks very straight forward, but either way I'm trying this, I still didn't get this to work.

Any help would be greatly appreciated.

Thanks in advance!