使用Google跟踪代码管理器在目标网页上设置document.referrer

Can you please guide me in setting up a Custom dimension which stores document.referrer only on the entry page of the website. So, this value remains same throughout the whole session. At the moment problem for me is Document.referrer is being updated with the new value of the previous page. Any help with clear explanation is highly appreciated.Can you please guide me in setting up a Custom