due to how recurrence flag works it's little tricky, but try following

- add single line text control on form, set its default value to NO and configure some javascript variable for it, eg RecurrenceFlagAlreadySet

if you want you can hide the control with formatting rule

- next configure a CSS class for recurrence checkbox, eg RecurrenceFlag

- finally add following javascript code into forms custom javascript setting

NWF.FormFiller.Events.RegisterAfterReady(function () {
  if ( (! NWF$('.RecurrenceFlag inputttype="checkbox"]').prop('checked')) && (NWF$('#'+RecurrenceFlagAlreadySet).val()=='NO') ){
     NWF$('.RecurrenceFlag inputitype="checkbox"]').click();
     NWF$('#'+RecurrenceFlagAlreadySet).val('YES');
  }
})
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