SendKeys method is only available on the window frame control. With chrome, you have to access the window frame through a proxy of the BrowserForm of the webpage. Refer to the screenshot below: 

How do I extract a proxy?

• https://help.roboticautomation.pega.com/80/Automations/Automations_-_Extract_Proxy.htm
• https://collaborate.pega.com/question/robotics-studio-how-create-proxy-object

UI Automation

• https://docs.microsoft.com/en-us/dotnet/framework/ui-automation/ui-automation-overview
• https://docs.microsoft.com/en-us/dotnet/framework/ui-automation/invoke-a-control-using-ui-automation

UI Automation Examples with a C# Script component

• https://collaborate.pega.com/discussion/automating-ie-download-bar
• https://collaborate.pega.com/question/not-able-interrogate-google-chrome-message-pop

Ah, my bad... I think you actually have to call a different SendKeys method. You can do it pretty easily with a C# Script... example is below. Use the handle from the iBrowserProxy. It will be a property on the control. Then pass that handle and the keys to the method.

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Windows.Forms;
using System.Xml;
using System.Windows.Automation;
using System.Runtime.InteropServices;
using OpenSpan.Adapters;
using OpenSpan.Diagnostics;
using OpenSpan.Utils;
using System.Windows.Forms;

namespace Dynamic.Script_8D5FD497EEED346
{
	// Script generated by Pega Robotics Studio 8.0.1100.0
	// Please use caution when modifying class name, namespace or attributes
	[OpenSpan.TypeManagement.DynamicTypeAttribute()]
	[OpenSpan.Design.ComponentIdentityAttribute("Script-8D5FD497EEED346")]
	public sealed class Script
	{
		//required to call the SetForegroundWindow method.
		[DllImport("user32.dll")]
		public static extern bool SetForegroundWindow(IntPtr hWnd);

		//This method will bring the window to the foreground, then sendkeys to the window. 
		//Ensure that focus is already set on the control that you want to sendkeys to.
		//The handle can be found as a property (or sometimes a method) on the control.

		public void SendText(string text, IntPtr handle)
		{
			if (handle == IntPtr.Zero)
			{
				MessageBox.Show("invalid handle");
				return;
			}

			//From the user32.dll - this brings the window into focus.
			SetForegroundWindow(handle);

			//System.Windows.Forms.SendKeys(string keys)
			SendKeys.Send(text);
		}
	}
}

Attached a working solution. Here you need two adapters as focusing "Choose File" control need to be handled by web adapter and passing Enter key simulation need to be handled by windows adapter. 

Steps to Configure both Adapters:

1. Create a Universal web adapter. In it's properties perform below changes.

• StartMethod = MonitorAll
• StartPage= https://cgi-lib.berkeley.edu/ex/fup.html
• HookChildProcesses= True

2. Create a windows adapter. In windows adapter properties perform below changes.(in "Arguments" property specify "StartPage" property that you have specified in web adapter)

• Arguments=https://cgi-lib.berkeley.edu/ex/fup.html
• Path=C:\Program Files\internet explorer\iexplore.exe
• HookChildProcesses= True

Steps to Start Interrogation:(always start universal web adapter first, then only windows adaptor)

1. Click on Start Interrogation button on Web Adapter. wait till bulls eye pops out.

2. Click on Start Interrogation button on Windows Adapter.

Note: Follow above steps in specified order.

You will be having two bulls eye windows for two adapters. 

3. Interrogate "Choose File" web control with universal web adapter bulls eye.

4. Then Interrogate Chrome frame where "Choose File" control is loaded with help of window adapter bulls eye.

Designing Automation:

1. Invoke Focus() method on "Choose File" web control, so that it is focused. 

2. Then Immediately invoke Sendkeys() with {Enter} parameter on chrome frame control in which "Choose File" web page is loaded, this simulation opens the window to choose file.

• Attached project developed in 8.0.2016.0