According to my research and testing, unfortunately, there is currently no way to use CSOM to include a custom message to users who see the access request page in SharePoint Online.
Thanks for your understanding.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.