You can drop a pin directly on a drawing by tapping "Drawings" from your tab menu and selecting a drawing from the list:
... or by tapping the New Post button, then picking the location/pin icon from the toolbar and selecting “Pin to drawing.”
Either way, you’ll be able to pick from your project plans, pan around and zoom in and then drop a pin on a specific location. You can do this by long-pressing on any location.
If your Project Organizer has added locations to your project plans the location will automatically be added to your post.
Note: Adding pins to drawings is only available from the mobile apps.