Sending websocket request to SignalR hub from Postman

I have a SignalR hub setup like this:

public sealed class NotificationUserHub : Hub {

    private ILogger Logger { get; }

    private IUserConnectionManager UserConnectionManager { get; }

    public NotificationUserHub(IUserConnectionManager userConnectionManager, ILoggerFactory logFactory) {
        UserConnectionManager = userConnectionManager ?? throw new ArgumentNullException(nameof(userConnectionManager));
        Logger = logFactory.CreateLogger<NotificationUserHub>();
    }

    public async Task<string> GetConnectionIdAsync() {
        HttpContext httpContext = Context.GetHttpContext();
        StringValues userId = httpContext.Request.Query["userId"];
        StringValues resourceId = httpContext.Request.Query["resourceId"];
        Logger.LogInformation($"Received connection request for user {userId} and resource {resourceId}");
        await UserConnectionManager.KeepUserConnectionAsync(userId, 
            Context.ConnectionId, resourceId).ConfigureAwait(false);
        return Context.ConnectionId;
    }

    public async override Task OnDisconnectedAsync(Exception exception) {
        string connectionId = Context.ConnectionId;
        await UserConnectionManager.RemoveUserConnectionAsync(connectionId).ConfigureAwait(false);
    }
}

I'm trying to test that this works in Postman. The client is able to connect but none of the requests are being received on the server. My guess is that there's a specific message structure required to interface with the hub but I can't find any documentation on it. What is the structure of the payload I should use to call GetConnectionIdAsync in my hub?

Update:

I attempted to implement it using this and I was, again, able to connect and authenticate with the server. However, I was unable to call the function, even after encoding my message in the format described here.