Drive LED with open-drain active-low signal
Depending on BKUPB current capabilities you could just connect the LED cathode to the port pin and the anode to a current limiting resistor and then to 5V.
Updated schematic to show ~1.5mA draw with a  2.5kΩ resistor.



simulate this circuit &ndash; Schematic created using CircuitLab
Below is the other common way to do it using an P-Channel enhancement mode MOSFET , and then you don't have to worry about the current draw of your open collector pin.



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