There's an elixir you can make from the alchemy station, but you need to be at least level 9 in order to learn how to make it. You can see if the elixir consignment store has it, but their stock is always random and they don't always have the elixir you're looking for. You can find another sim in town who knows alchemy, and have them make the elixir and give it to your sim.
If you don't mind cheating, you can also hold down Ctrl+Alt+C, type "testingcheatsenabled true" (no quotes), then hold down Shift, click on your sim, and select "Edit in CAS", where you can change their life state to Witch.
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If you don't mind cheating, you can also hold down Ctrl+Alt+C, type "testingcheatsenabled true" (no quotes), then hold down Shift, click on your sim, and select "Edit in CAS", where you can change their life state to Witch.
Its called the witches brew potion
You must create elixir for that, on alchemy station.