With an anonymous function:

SDG.Publisher.registerSlot('banner', document.getElementById('myBannerContainer')).load().nativeBackfill(function() {
  var ad = document.createElement('div');
  ad.style.width = '728px';
  ad.style.height = '90px';
  ad.style.border = '1px solid';
  ad.style.backgroundColor = 'red';
  return ad;
}).load();
 
 With a declared function:
function myAd(){
  var ad = document.createElement('div');
  ad.style.width = '728px';
  ad.style.height = '90px';
  ad.style.border = '1px solid';
  ad.style.backgroundColor = 'red';
  return ad;
}
 
SDG.Publisher.registerSlot('banner', document.getElementById('myBannerContainer')).loadnativeBackfill(myAd).nativeBackfillload(myAd);