The KeyStrokes function reads a string which itself consists of the keystrokes you wish to execute.

The CDF description:

Function Name: Keystrokes
CDF Library Name: demacro.dll
Return type: Int

One Parameter:
-Name: KeysString
-Type: String

The button action should be set to [Execute CDF] and the argument (including the quotation marks) is: KeyStrokes(“_tab”) or, if a combination of keystrokes is required, the argument is KeyStrokes(“_shift(_tab)”).