$Uri = 'http://httpbin.org/headers'
Invoke-WebRequest -Uri $Uri -UserAgent 'Invalid:Agent' -SkipHeaderValidation
Invoke-RestMethod -Uri $Uri -UserAgent 'Invalid:Agent' -SkipHeaderValidation

Invoke-WebRequest : The format of value 'Invalid:Agent' is invalid.
At line:2 char:1
+ Invoke-WebRequest -Uri $Uri -UserAgent 'Invalid:Agent'
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Invoke-WebRequest], FormatException
    + FullyQualifiedErrorId : System.FormatException,Microsoft.PowerShell.Commands.InvokeWebRequestCommand

Invoke-RestMethod : The format of value 'Invalid:Agent' is invalid.
At line:3 char:1
+ Invoke-RestMethod -Uri $Uri -UserAgent 'Invalid:Agent'
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Invoke-RestMethod], FormatException
    + FullyQualifiedErrorId : System.FormatException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand

StatusCode        : 200
StatusDescription : OK
Content           : {
                      "headers": {
                        "Connection": "close",
                        "Host": "httpbin.org",
                        "User-Agent": "Invalid:Agent"
                      }
                    }

RawContent        : HTTP/1.1 200 OK
                    Connection: keep-alive
                    Date: Thu, 03 Aug 2017 10:25:25 GMT
                    Via: 1.1 vegur
                    Server: meinheld/0.6.1
                    Access-Control-Allow-Origin: *
                    Access-Control-Allow-Credentials: true
                    X-Powered-...
Forms             :
Headers           : {[Connection, System.String[]], [Date, System.String[]], [Via, System.String[]], [Server,
                    System.String[]]...}
Images            : {}
InputFields       : {}
Links             : {}
ParsedHtml        :
RawContentLength  : 113
RelationLink      : {}


headers : @{Connection=close; Host=httpbin.org; User-Agent=Invalid:Agent}