Access Token option addition to libcoap about libcoap

[obgm]

To clarify: The token in a CoAP message is used to match a CoAP response to the CoAP request that has triggered this reponse. By this, it is inherently tied to the CoAP messaging layer.
According to your description it seems that you want to try to pass an authorization token (sometimes inaccurately called access token) for some application protocol. This is transparent to CoAP and therefore, libcoap has no direct support for this (although you can retrieve the token in the CoAP PDU in the request handler to achieve what you are trying to do). Your scenario warrants for using a POST request to convey your token.

As a side note: There is a series of IETF specifications that detail how to use CoAP (or other REST-friendly transport protocols) to not only transfer authorization tokens but also create a security association between communication peers based on this token. This is described in RFC 9200 ff. There is also an implementation of the DTLS profile based on libcoap.