I thought the order should be "en penna till." That's how it was when we learned it with "ett fönster till." I'm also hovering over en till and it says it means another. Would "Har du en annan penna" work?
Both word orders work, I'd personally prefer En penna till, but both are OK. en annan penna would mean more in Swedish than another pen in English, it would mean a different pen.
Shouldn't "En extra penna" work as well
"En penna till" was not accepted for me...