"My last name is Zhang, and yours?"
A solution for this case is put the mouse cursor over the "Zang", then it will show the symbol "张". Select and copy it, then make the sentence with all the other symbols, but don't press the "check" button. Instead, press the "use keyboard" button, then paste the missing symbol in the right place.
Why don't we need the character showing possession or the linking 'be' particle here, but it's required in the "what's your name" translation items?
The only issue I see with that is people using only the pinyin and never using the characters. I'd hope people would at least try to learn them simultaneously. That being said, I didn't pay attention to tones for the first half of my studies on another site, and now I'm paying for it.