How to read signal on Arduino?

Question

Arduino

There is a Digispark board with an ATiny85 controller.
There are 2 power supplies connected to the board.
1. The board itself is powered by the USB connector of the PC.
2. At the 5th input, + 5V is supplied from another source, and the minus is also connected to a common ground.

pinMode(5, INPUT);
INT signal = digitalRead(5);

I switched the controller input to the Arduino IDE into the input mode, and after that I read the input readings.
The adapter constantly receives a 0 signal from this output, but with a multimeter I see that power is supplied to the 5th leg.
How to properly connect the receipt of a signal from the second IP. Maybe throw a resistor to ground from the input. or diode?