I went to a free advertisement website ans finished all the steps and it says this:
After you Add Your Site to the Network (see 'So I registered...Now what?' if you need help doing this), we will present you with some Ad Code that you will need to put in the source code of your website. The Ad Code is about 10 lines of Javascript that will automatically fetch related advertisements and display them on your website in the way that you indicated when you Add[ed] Your Site. By displaying ads on your site, you tell our servers that you are actively participating in the Network and we will in turn display your advertisement on other similar pages.
I have my ad code but where do I go the get to the source code of my fourm?

Just add the code to Display>Generalities
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Thank you for your help but I decided not to do this anymore