Which route?

If it's any MH codeshare segment *except* DOH-KUL, and it's all on one QR booking, then you can open the whole thing on the MH website/app and select seats and meals on all your segments, whether they are operated by MH or not - full functionality, super smooth

For reasons best known to MH and QR, if it's QR-coded, MH-metal DOH-KUL, then you're passed from pillar to post, and QR tells you to call MH and vice versa. Lots of threads on the QR and MH boards to this effect. My best guess is because of the slightly convoluted seat allocation policy between MH and QR coded pax on these flights - see also AY-QR codeshare flights. However the QR booking ref comes live on the MH system at T-48 hours so you can check in and reserve seats then. If it's any consolation, nobody else can reserve seats either, so you should have your pick

I have tried all the other OW MMB websites, and... same problem: can't access DOH-KUL

Edited to add: someone seems to have a workaround, but I've not been able to get this to work in the past: Reserving seats on MH flight ticketed by QR