The Two-weapon Fighting rules (Player's Handbook, page 195) are quite clear about the requirements for making the off-hand attack:
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 cannot use Two-weapon Fighting without taking the Attack action. Other actions do not count.
Notably, since Two-weapon Fighting requires both weapons to also be light melee weapons, darts cannot be used - they are listed as ranged, not melee weapons.