Selecting a product variation attribute

[dmainmon]

I can add product attributes and product variations. How do you select the default attributes for a variation? The top row of the image below shows what I'd like to achieve. The options for the drop downs are set in the product attributes.

I've tried the following code:

 var variationList = new List<Variation>();
 List<ProductDefaultAttribute> default_attributes = new List<ProductDefaultAttribute>();


 variationList.Add(new Variation
  {
       regular_price = 33.3,
       price = 23.99,
       description =  "Some product " ,
       weight = 1.2,
       sku = "SKU num"
      // attributes = myVarAttrList

   });


`List<VariationAttribute> myVarAttrList = new List<VariationAttribute>();
        VariationAttribute varAttrib = new VariationAttribute();
        varAttrib.option = "Not a Sample";
        varAttrib.name = "Sample";
     //   varAttrib.id = 2;

        myVarAttrList.Add(varAttrib);

        foreach (Variation variat in variationList)
        {
          //  variat.attributes.Add(varAttrib);  this causes and Object reference not set exception
            variat.attributes = myVarAttrList;

            var addVar = wc.Product.Variations.Add(variat, Convert.ToUInt32(RecID));

          
        }`