COAP Secure implementation


I am trying to use secure COAP and got an error when compiling: undefined reference to 'otCoapSecureSetPsk'.

This function is declared in "coap_secure.h". I am digging in to the header file, and it mentioned:

The functions in this module are available when CoAP Secure API feature (`OPENTHREAD_CONFIG_COAP_SECURE_API_ENABLE`) is enabled.

In prj.conf, I already added CONFIG_OPENTHREAD_COAPS=y.
But in source file, coap_secure_api.cpp, looks like I also need to define MBEDTLS_KEY_EXCHANGE_PSK_ENABLED in order to activate this function. However, I still got the same error when build the project.

Can you please show me how to declared this configuration, as I cannot find the correct configuration key?

Please feel free to let me know if you have any suggestions.

Best regards,