We understand the reason for wanting to personalize a message.
However, at this time it is not possible. The system has been designed to make it simple for all users, as well as compliant with company's policies.
When you send the opt-in email, you should have had a conversation with your prospect and they should be expecting an opt-in email.
When talking to a prospect, please make sure to tell them that you will send an email confirming that you have the right email, and they will have to click on the link in order to receive emails containing information on your company.
You can always follow up by emailing a resource and personalizing that email.