Sometimes you just need to throw in a piece of invisible tracking HTML/Script that you've received from a third-party. While it is possible to add this to your page by writing a new Custom Code Widget, there is a much simpler and robust way to do this.
Below is an example of such an invisible tracking code.
<img src="https://track.adsome.code/adfserve/?btk=6666;1x1invisible=1;srctype=3;ord=12412515" width="1" height="1" border="0">
To add this tracking code to your published project, go to the Publish Settings of your project and click on the Settings icon.
Click on "Show Advanced Settings" and add the package "Custom HTML" to your project. Then write the code into whichever is appropriate. Here we have added it to "Start of Body" which means the code will be injected right before the actual content of the page begins.
Most third-party providers will instruct you on where your code should be put on the site. If they do not, add it to the start of Body to be sure (test and see if it works).