The answer was not obvious but it seems you can't just input test on the focused element. You have to specify the element by name. I've used it's ID.
On my web page the ID of the element is 'userInput', so you use:
Private Selenium as Pnd_Selenium (for example)
Dim WebElement1 As Object = Selenium.FindByIdWebElement("userInput")
Selenium.WebElementSendKeys(WebElement1, "Text I would like to Input")
I hope this is helpful for anyone else.