After SendKeyStringToHostPlusEnter method as per your screenshot, add another SendKeyStringToHostPlusEnter method and it's key values should be "exit". Generally to close command prompt "exit" is the command, above suggestion was to implement the same.

After the SQLCMD have been executed, but still the cmd prompt was not getting killed, if this is the scenario, you may use below C# script to kill the cmd prompt process i.e below code is equivalent to killing cmd prompt from task manager.

You need to provide the process name as in task manager.

C# code:

public String kill()
       {
           String WasRunning = "";
           Process[] cmd = Process.GetProcessesByName("CMD");
           if(cmd.Length == 0)
           {
               WasRunning = "false";
           }
           else
           {
               foreach(Process p in cmd)
               {
                   try
                   {
                       if(!p.HasExited)
                       {
                           p.Kill();
                           WasRunning = "true";
                       }
                   }
                   catch(Exception ex)
                   {
                       WasRunning = String.Format("Unable to kill process {0}, exception: {1}", p.ToString(), ex.ToString());
                   }
               }
           }

           return WasRunning;
       }