Use .outerHTML to get the HTML

Here is a code sample to get the entire HTML of the website

private async void GetHTMLAsync()
{
    var siteHtML = await webView.InvokeScriptAsync("eval", new string[] { "document.documentElement.outerHTML;" });
}

Use .innerText to set the value

Here is a code sample to enter text in Search Box on Bing website

private async void EnterTextAsync(string enterText)
{
    var functionString = string.Format(@"document.getElementsByClassName('b_searchbox')[0].innerText = '{0}';", enterText);
    await webView.InvokeScriptAsync("eval", new string[] { functionString });
}

Use .click() to simulate click

Here is a code sample to click search button on Bing website

private async void SimulateClickAsync()
{
    var functionString = string.Format(@"document.getElementsByClassName('b_searchboxSubmit')[0].click();");
    await webView.InvokeScriptAsync("eval", new string[] { functionString });
}