Please note if you are using your own thank you page this is not going to work, in that case you have to add the pixel on your own page.
To add pixel for your thank you page, go to your event editor, then to "Miscellaneous", then click on "Configure" for "Tracking Code Setup".
Find "Thank You Page"
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".