If you're gaining consent via email your client will receive a message in their inbox with a "Review Consent Form" link in it. When they click on the link they will be taken to a secure (non PII) portal where they can review your consent form and provide their consent. Here's an annotated example of what they will see.

When you create a consent form we have given you the ability to specify some introduction text at the top of the portal page where you can explain why you have requested consent and what they need to do.

What you include in here is totally up to you, but something along the following lines would work if  you're seeking to get consent in accordance with Article 14 of GDPR.

"GDPR (General Data Protection Regulation) is the most significant piece of privacy and data protection in twenty years. It takes effect on 25th May 2018 and from that date we are required to ensure that we gain a new data protection and privacy consent from all clients. In it (amongst other things) we confirm what information we hold about you and how we are permitted to use it. Once you have reviewed the consent form below please confirm your relationship to the client (if you are not the client) and click on the "Consent" button."
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