what can i substitute the dried marjoram with?

One can use dried savory, dried sage, maybe celery seeds would be ok.

This recipe includes marjoram, because it's supposed to be Italian, from the movie The Big Night. My own thinking is that marjoram and (even more so) oregano aren't that great with smoked salmon. If you like it, I'd choose dill. If you don't like dill, just pick something you do like, or go really safe, and use parsley.

Oregano & Marjoram are very similar.

I've never done it, but it sure smells a lot like oregano.