The Reviewer's receipt is usually filled by the reviewer to notify the editorial office whether he/she would like to review the manuscript. It is important for the editorial office to schedule the reviewing process.
The Reviewer may click on the URL contained in the Email to fill the reviewer's receipt online. Meanwhile, the reviewer may suggest several alternative reviewers if he/she could not do the reviewing.
Reply to the email and inform the editorial office your reviewing choice is also a good way, but it may have some delay, because we have to fill the receipt for the reviewer manually.