How to add a string of characters in the "replyto" field, for example: John Doe jd@domain.com.

Thank you for your help

The notification email is sent to the owner of the web site filled in with the data from the form that the visitor filled out. I'm assuming that is you. When that email comes to you, if you click reply, the email will automatically be addressed to whatever the Reply-To address is. Therefore, you don't need both the name and email. You only need the email address.

When you look in your notification settings, there is a drop-down to select the Reply-To email. Make sure your form has an email field placed on it in which the visitor can give their email address. Once you do that, the email field will appear in the drop-down list for the Reply-To.