The Two Weapon Fighting bonus action attack requires that you take the Attack action and attack with a light melee weapon.
The rules for Two Weapon Fighting state:
When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can use a bonus action to attack with a different light melee weapon that you're holding in the other hand. You don't add your ability modifier to the damage of the bonus attack, unless that modifier is negative.
The Bladesinger's Extra Attack feature reads:
You can attack twice, instead of once, whenever you take the Attack action on your turn. Moreover, you can cast one of your cantrips in place of one of those attacks.
So when you do (attack + cantrip), you are taking the attack action and the first attack is with a light melee weapon. Therefore, you would qualify for the bonus action attack from Two Weapon Fighting.