Ok dear, the first thing is that you have no authority to send this email to your boss to request the power to perform this job. However, if he has asked you to send him a draft email, then yes, you can send it. But be careful, he is your boss, and no junior employee should send this type of email to their boss. This could raise many questions for you, even though your boss requested it.
It is not difficult to write a letter when you know what you are going to do and have to include. The letter should be simple and straightforward so that all employees can consider it an official notice for the same purpose. Now, the solution to your problem is as follows:
Dear Folks,
With reference to Mr. A (your boss), this is to inform you that you are all required to submit your future medical leave applications to me for approval, effective from [date]. I have been designated as the authorized person for this purpose. Therefore, you are requested to contact me and submit your medical certificates accordingly.
Sincerely,
Your Name