This code snippet will write to registry on  a client side.
 (Only works in IE!)

 

 function writeInRegistry(path, sRegEntry, sRegValue) {

        var regpath = path + sRegEntry;

        var oShell = new ActiveXObject("WScript.Shell");

        oShell.RegWrite(regpath, sRegValue, "REG_SZ");

    }

Notes:
1. On the client side, please make sure that the UAC(User Account Control Settings is set to Never Notify)

• Internet Options
• Security
• Custom Level
• Look for ActiveX controls and plug-ins
• Mark enable option under Initialize and script ActiveX Controls not marked as safe for scripting(not secure)

• Mark enable option under Script ActiveX Controls marked safe for scripting.

• Click  Ok