1) Find an open spot in the Inventory view
2) Click "Hold" button on any open position
That will open up a "Hold Inventory" modal you can use to create a Hold.
3) Add in any necessary details
You can assign the Hold to an Advertiser you already have in Megaphone. You can press "-" to change the Expiration to "NONE" so that the Hold stays permanently or you can choose to keep the Hold for a set number of days.
You can also add any other relevant information in the "Notes" field.
4) Press "Place Hold" and view Hold on Inventory
Here's how the Hold appears on the Inventory view after you've created it.
5) View "Details" of your Hold
After a Hold is created you can view the details, including the user that created the Hold and when they created it.
You can also press "RELEASE HOLD" to remove any Holds you don't need/want anymore.