An imperative has no subject, so its passive is built with 'let' plus 'be'.

The passive needs a subject and a verb; an imperative supplies neither. 'Let … be …' is the frame English uses to keep the command force while removing the doer.

Close the gate. → Let the gate be closed.

'let' supplies the missing frame

The gate is closed.

a statement, not a command

Where it bites

The alternative form 'you are requested to…' is also correct and is often the key where the sentence is a polite request rather than an order.