Please note: This will only work when you do not have any redirect setup.
To add pixel for your completion page, go to your event editor, then to "Miscellaneous", then click on "Configure" for "Tracking Code Setup".
Find "Webinar Completed"
The first box is for placing the pixels inside <head></head>
The second box is for placing the pixels just after </head> i.e head tag closes.
The third box is for placing them just before </body> or end of body tags.
Generally pixels are placed inside head unless you are specifically told.
Once you place your pixels, don't forget to hit "Save".