With Kestrel I can do the following:

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*",
  "Kestrel": {
    "EndpointDefaults": {
      "Protocols": "Http1"
    }
  }
}

to make it run in HTTP1.1 mode.

How can I achieve the same with IIS Express? At the moment it is defaulting to HTTP2