score:1

Accepted answer
foreach(var key in dictionary2.keys.where(k => !dictionary1.keys.contains(k)))
{
    dictionary1.add(key, defaultstring);
}

score:0

var dictionary3 = dictionary2.todictionary(x => x.key, 
    x => dictionary1.containskey(x.key) ? dictionary1[x.key] : "def2");

score:0

var allkeysindict1 = dictionary1.keys;
var allkeysindict2 = dictionary2.keys;
var missingkeysindict1 = allkeysindict2.except(allkeysindict1);
foreach (var missingkey in missingkeysindict1)
{
    dictionary1.add(missingkey, def2);
}

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