Adds a custom header to send with outgoing messages.
async stomp.addHeader(key : string, value : string): Promise<void>
This method is used to add user-defined headers to the list of headers held by the Header* properties.
Note that the STOMP specification defines a number of standard headers necessary for implementing the STOMP protocol. When constructing an outgoing message, the class silently ignores any user-defined headers that are already set by the class.