urxvt-selection-to-clipboard man page
selection-to-clipboard — copy the selection to the clipboard each time a selection is made
urxvt -pe selection-to-clipboard
This very simple extension copies the selection to the clipboard every time a selection is made. This, in effect, synchronises the clipboard with the selection for selections done by rxvt-unicode.
urxvt256c-ml-selection-to-clipboard(1), urxvt256c-selection-to-clipboard(1) and urxvt-ml-selection-to-clipboard(1) are aliases of urxvt-selection-to-clipboard(1).