When sharing a file with an external user, SharePoint will generate a unique URL specific to that user. You can use the "Copy Link" icon on the bottom of the Send Link dialog to copy the link to your clipboard, and then mail this link to the user from your email account. We strongly recommend not using the Send button because it's very possible that the auto-generated email from SharePoint can end up in the recepient's spam folder.
Once the user receives the URL (either from you or from SharePoint), they will click on the link to open the file.
But before they can get to the file, Microsoft wants to ensure that they are the intended recepients. Remember, the link is specific to the user to whom it was sent. This is done to avoid the situation in which a user forwards the email to someone else, which was not something you intended. So they will be prompted with the screen to send a verification code to the email address for which the original sharing was intended.
Clicking the "Send Code" link will automatically redirect to a page that looks like the one below left and send the user a code (sample email below right). It is important to tell the users to always check the Spam email folder, as our testing has shown that the automated messages from Microsoft often end up in the Spam, especially if the external email is Gmail.
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