Non-EU nationals who do not hold a C permit have no automatic right to family reunification. It's down to the individal cantonal authorities to decide whether or not they'll grant a dependent's permit. https://www.ch.ch/en/family-reunification/
It's not impossible to get permission, but they will look at your relationship closely to make sure this isn't a sham marriage to get your partner into the country and get a permit for them. As you've been together for so long though it shouldn't be a problem to provide things like holiday photos, joint rental contracts, etc, which you may be asked for.
Also what is your partner going to do when they get here? Do they expect to be able to work? Because if you end up with an L permit then there are restrictions on dependents working. Unless they have professional qualifications (i.e a university degree) then they will not be able to work here. If you get a B permit then the restriction doesn't apply.