On YW163, there is the creature feature Venomous that states:
Your claws are venomous. Make a Fists maneuver; if successful, the
target gains a Poisoned aspect.
In each subsequent exchange, the
target must roll Endurance to defend against an attack from the poison
equal to your Fists score. Once the target concedes or is taken out
(falling unconscious), the attacks stop (see page 203 for guidelines
on being taken out). However, the damage is already done; without
proper medical attention, a taken out victim will die soon (within a
few hours, perhaps less), though not immediately.
attention will remove the aspect and end the effect. This is an
opposed roll—you can roll Fists (since that was the skill for the
original attack) to set the difficulty to mitigate the poison.
In addition, on YW326, the following text applies:
Some weapons (like tazers or some poisons) are not necessarily
intended to do direct damage as much as incapacitate the target. These
weapons should still get ratings based on the guidelines given in
Playing the Game (page 202), but instead of applying the Weapon rating
as stress on an attack, the attacker might instead opt to impose a
temporary aspect on the target (as though he’d performed a maneuver)
in addition to the stress from the attack roll. Weapons used to bind
or capture, like nets, can have their ratings sacrificed to enter the
target in a grapple (page 211) in addition to the inflicted stress.
either case, rolls to overcome these secondary effects are made
against the weapon’s rating; use the rating as the basis for rolling
the opposition—i.e., Weapon:2 = Fair (+2).
So, taking the two together, I'd make a regular weapon attack, and apply the Poisoned aspect if a consequence is taken. Then use the guidelines from Venomous to take over from that point, using the rating for the poison.