esp-idf/examples/protocols/coap_server/main/Kconfig.projbuild

menu "Example CoAP Server Configuration"

    config EXAMPLE_COAP_PSK_KEY
        string "Preshared Key (PSK) to used in the connection from the CoAP client"
        depends on COAP_MBEDTLS_PSK
        default "secret-key"
        help
            The Preshared Key to use to encrypt the communicatons. The same key must be
            used at both ends of the CoAP connection, and the CoaP client must request
            an URI prefixed with coaps:// instead of coap:// for DTLS to be used.
endmenu