"The target project openSUSE:Backports:SLE-15-SP6 is not accepting requests because: Please submit to openSUSE:Backports:SLE-15-SP6:Update"

Now this is not good either, since I'm not trying to submit an update but a new package.

Actually this is exactly what you would need to do, an "optional" update that makes the package available. However..

So what should I do instead to include a package in Leap?

Updates that add packages to openSUSE Leap need to fix bugs. For example if it is genuinely missing when it should be there, or it replaces an earlier version that became unmaintainable. Things like that. That excludes "it's not there and I was late adding it to the distribution before it was released". Not that we did not ever do this, but it needs to be important.

Andreas