As far as I know, a referendum can take two forms: either it's the final text or it simply is a mandate to create a law with certain parameters, the lawmakers can then decide on the exact wording.
1. An addition to the constitution. The parlament then has to implement that into law.
2. A referendum on a law that has been passed by parlament. This allows the citizens to revert a parlamentary decision.
There is no way to vote on a new law directly. However you can get close by just having a very detailed constitutional change that doesn't leave a lot of options for implementation open.
