You can insert an Opt-In form into an email sent to clients, but they will not be able to subscribe from there. Email providers do not allow subscription via Opt-In forms inside the received email messages for security reasons, just as they do not allow playing of embedded video and audio materials inside email messages.
What you can do instead is to create 1-click opt-in links in the email you send (for the Opt-In forms you want your contacts to subscribe to). Then your contacts simply need to click that opt-in link to subscribe, be taken to the Thank You Page of the Opt-In Form and start receiving the messages in the autoresponder series associated with the Opt-In Form.
A tutorial about the 1-click opt-in links:
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