It doesn’t exist. I have the proof. Look. It’s missing.
This is a fairly common gap for languages to have, though it's not universal. (Ancient Greek, for example, has regular imperatives in the active, middle, and passive voices.) So it's not surprising that Swedish lacks this form.
The reason it's so commonly missing is because of the semantic oddness. Normally an imperative is commanding someone to do something, while the active voice represents someone doing an action: it fits well. In formal terms, you're giving a command to the agent, who has the power to do the action.
But the passive represents something being done to someone. In formal terms, the subject of the sentence is not the agent. Since the person being commanded isn't the agent, the verb is generally not representing an action under their control. So it doesn't always make sense to use an imperative.