Though many promos are intended to run in an exact position in an episode (i.e. the second Mid-Roll spot), promos can also be set to run as positionless. When "No Position" is selected, a promo will insert into any open position within the selected unit type (Pre-, Mid-, or Post-Roll) wherever available.
To set up a positionless promo, start by Creating a Promo or selecting an existing promo from the Promos tab in the left sidebar.
Click into the desired Position and select the "Delivery" tab at the top. In the "Delivery Details" section, mark "No Position" in the "Position" category.
Doing so will allow the promo to run in any open position for a designated unit type. For example, if the promo has been selected as a Mid-Roll location with no position, Megaphone will insert the promo into any open Mid-Roll spot (be it the first Mid-Roll, second Mid-Roll, etc).
In order for the ad-insertion to behave as described, be sure to have ad locations defined in order for the promo to run!