What are you trying to copy and paste? Rather than getting into
operating system specific actions, you might want to see if you can do
the same thing with Selenium or javascript commands. For example, if
you are trying to get the text inside a web element, get the
WebElement than save the element.getText(). Same thing as a copy. The
paste then does a sendKeys with the string from the element.getText().
This is Java but the same idea should work with other bindings, The
syntax will be slightly different.
If you are trying to copy the value from an <INPUT> element then you
want to use element.getAttribute("value"); rather than getText().