I use Microsfot.AspNetCore.SignalR.Client 7.0.7 and .NET 7 to implement a Real-time Chat App. I want to test the SignalR in Postman. I found a good article here. I followed the steps and it worked as expected.

1. Postman supports WebSocket protocol. You can use Postman to test WebSocket APIs.
2. The below image show how to connect to a SignalR API using Postman. Click 'new' button, then select 'WebSock' in the pop-up window.
3. This below message is telling SignalR server which message format for the communication and every message needs to have a 0x1E/U+001E character at the end of the message which is unseeable character and hard to type.

   {"protocol":"json","version":1}
   								

   If Connection is successful, you will get a response from server like this in the red box.:

In conclusion: The message sent from client to SignalR server should end with an unseeable special character.