When you apply for an affiliate program, they ask you to put a verification code in the header.

Before you insert the code - select all - copy - and save it in a notepad- in case it does not work out the way you thought and have to change it back.

The easiest way is to activate the Header and Footer Scripts plugin, you get access through this plugin to your header. But if you insist to get your code directly into the editor, here is how:

Go to appearances - click on editor