primary and secondary menu aria-hidden:"true"

Hi Team,

to make the menu compliant to the Accessibility standard in Austria I must achieve the following:

If a user is navigating just by tab the focus shouldn´t automatically jump to the submenu, just on enter, the submenu should be accessible. I thought i could achieve that by adding aria-hidden="true" to the sub-menu-item, but unfortunately, I don't know how!?!?

I hope you can help me or give me a hint ;-)