I don’t know about Masters, but I did a Doctorate on applying behavioral science to interactive design and digital campaigns, so here’s some advice.
Going cross disciplinary can be a challenge because of institutional politics and formal traditions. You’ll hear great things about interdisciplinary studies, but quite often, you’re punished for crossing lines, with ambiguous fit.
Your best hope is to try and find a program that has some of the components of both, and settle for an imperfect fit, that will hopefully cover enough ground.
You can either go for full HCI, and backfill with psychology. Or apply for a psychology program, that has scope for HCI.
I specialize in applying psychology to tech, and there are a number of academics with faculty positions who are passionate about this topic, but they only teach a few classes on this subject. You may find them in AOIR or in the Persuasive conference network, and in some cases, they can support you through a custom program, but this is rare at the Masters level, so it’ll be based on lots of shopping around. AOIR is probably a great network to ask for help.
At the masters level you’ll have fewer levels, as you essentially need to find a program that’s exactly on the spot. A MPhil or Doctorate will allow you more options to tailor your own study.
Whatever path, you can always make this your Masters thesis, and then try to align your focus on a topic that hits your passion point, blending psychology and HCI. If you can find a program with a two credit thesis program, you may be able to pick your courses, to focus on a thesis that’s totally on the mark.
For formal HCI, there’s no shortage of psychology, but it just extremely heaving on cognitive psychology, and totally ignores other fields, like social psychology, persuasive psychology, motivation, and I think neuroscience is more interesting, as I believe that all HCI/UX will be guided by neurobiology in the next while. So you’re in a difficult space, as you’re ahead of the market, so you’ll have to be creative.