I've purchased 016 tickets on the UA website with native LH segments on them, but I've always had at least one direction of the itinerary on UA (for example, LH JFK-FRA outbound, UA MUC-EWR return). If that's the case, you can find the flights on Hipmunk or Google Flights and click through to United for a 016 ticket purchase. If you have no UA segments, then it's easiest to get a 016 ticket if you use UA flight numbers for codeshare segments, but you may be able to call and purchase the itinerary by phone.