Thanks in advance to any more experienced international travelers that can help. I don't fly internationally hardly at all, so I'm not sure how the following works...

I have a trip where the return is OPO-FRA on LH and then FRA-xxx on United (to return back to the US). My understanding is that since the originating segment is on a codeshare, I will have to check-in with LH, in particular.

Is it generally possible to do this on-line via LH's app? Or is it necessary to be physically present at the airport to accomplish the check-in? (I will have *no* luggage to check, BTW.) My concern is that it is a 6 AM flight out of OPO, and if I have to complete the check-in "in person", then I am thinking I may have to do that the preceding day since I'm not sure how early in the morning LH staffs their desk at OPO.

Also of relevance, perhaps, is that when I check the LH reservation on LH's website, it claims that I need to enter my passport data. When I try to do this I get an error. So, I am wondering if I need to call-in to have this resolved. I'm not sure how this may affect any check-in issues.

Anyway, thank for any advice. I'm trying to have all my bases covered and not have an unpleasant surprise when I try to get back home.