I have a Travel Voucher Exchange Coupon.

I'm trying to book GEG-DEN-ORD-AMS (UA) with AMS-FRA-DEN-GEG return on LH besides the last segment.

They said this is not allowed because the itin is partially on codeshares. Also, UA.com won't let me hold the ticket because its a promo fare.

Is there a way to type in Travel Voucher numbers online or something. The only thing she could find is a fare double the price on UA metal.