The Unisystem VS has an input normally connected to a coin acceptor. Electrically, the coin acceptor is similar to a button.
The game monitors bit 5 at address $4016 (same address as first controller) for pulses coming
from the coin acceptor. When this address ($4016) is read, cpu pin #36 goes low. When this occurs,
the 74hc368 (U7) takes control of a few data signal on the BUS. D5 however, is left floating.
In order to drive D5, we can use a spare gate of U7. Cut the track connecting pin #6 to ground
and install a pull-up resistor of about 10k on it. Next, connect this pin to a momentary switch (or to
a real coin acceptor) which will connect to ground when pressed. Finally, connect U7 pin #7 to the CPU
Here is a picture explaining those steps:
Unisystem VS schematic and the
NES input circuits schematics for more details.