Syntax for Custom PHP field

Trying to pass a url to curl within the Custom PHP field and keep getting errors on execution. Not sure how to interpret the field instructions “You must put your >?php tag to initiate PHP where you want it used”. I do see your script runs an eval function against the code snippet but unsure what php tags should surround the code. Could you provide an example? Here is the kind of thing I have been trying:

<?php curl "https://example.com/wp-content/plugins/plugin/script.php"; ?>