request, send a message
囑 centers on conveying a need or instruction to someone else: making a request or sending a message to ask for something.
囑 combines 口 (mouth) with 屬, which likely contributes the sound and the idea of connection or attachment. Together they suggest using words to connect with someone and convey a request.
A mouth 口 beside 屬, a shape associated with linking or attaching. Picture your mouth forming words that link you to another person, sending a request across the connection.
For ショク, imagine sending a request for a shockingly large meal: shock -> ショク, and your mouth asks for it.