Channelback HTML message

Using the channelback feature : https://developer.zendesk.com/api-reference/channel_framework/understanding-the-channel-framework/channelback/

 

I am not able to answer to a ticket with html message.

Documentation : 

message

string

65535

The agent's reply

 

 

Is there any way to add some html in the response, or at last breakline?

 

Thanks

BR