How do I track models or brands in piwik C# api?

Using the C# api, how to insert the device brand or model information to the user agent?
I want to achieve something like:

int osMajor = 6;
int osMinor = 2;
string strBrand = "Dell ";
string srtModel = "Inspiron";
string UA = "Mozilla/5.0 (Windows NT " + osMajor + "." + osMinor + "; WOW64; en-US;"  + strBrand + srtModel + ")";

Hi, the UA string is something like

Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.2; WOW64; Trident/7.0; .NET4.0C; .NET4.0E; .NET CLR 2.0.50727; .NET CLR 3.0.30729; .NET CLR 3.5.30729; Creative AutoUpdate v1.41.07; Microsoft Outlook 14.0.7188; ms-office; MSOffice 14)

In general, why do you need to forge this information? It is sent by the browser directly.
I anyway think that the UA specifies the hardware info especially for mobile devices.
Anyway there’s a website that may help you find a solution
http://www.useragentstring.com/