ログインユーザのフォーム送信

I am always indebted.
Please forgive me because I may just not know how to do it.
How can I submit the form while logged in and include the logged-in user’s ID in the submitted content when the administrator receives the form input contents by e-mail?
In other words, we want to identify which user has registered as a member.
The version I’m using is 8.5.5.
I look forward to working with you.