I’m not aware of a mechanism to do that cleanly. 

Some workarounds I can think of:

You could add it to the templates in the custom email message portion. (but won’t show up for non-templated envelopes)

You could include that text in one of the footer/headers of the emails in your Brand settings.  (not sure how that looks to the end-users though)

Hello ​@Jalal,

Thank you for reaching out, and welcome to the Docusign Community! We appreciate your question and assure you that we are fully committed to providing you with the best service possible. 
To customize your email envelope message, please check this article Set the Recipient Language and a Custom Email Message.

1. Start a new envelope and prepare it as usual, adding documents and recipients.
2. Select Custom email and language for each recipient in the Add message section.

   Note: You can personalize the content sent to recipients when customizing email messages. However, in the envelope details view, the message section will show "No message has been entered." A message will only show on the envelope details page if you enter it without selecting the customization option.

   

   The message section expands to include a message setting for each recipient. By default, the email subject for each recipient is populated with "Complete with Docusign:" followed by the file names of the documents in the envelope. This field is limited to 100 characters; anything longer is truncated. You can edit the email subject as desired.

3. Set the email language for each recipient. The language setting controls the set portions of all email notifications. The message subject and body are not translated. Whatever you enter into these two fields is transmitted exactly as entered.
4. Enter a custom email message for each recipient. The text is not translated. It is sent to the recipient exactly as entered.

   

5. Complete and send your envelope as usual. Your recipients will receive an email notification inviting them to sign in your chosen language. Example:

   

6. In your account, the envelope takes the name as entered in the email subject for the first recipient listed on the envelope.

   

Here’s what your signers will see on their end, and they may click on “Other options” to select “Assign to someone else.”
 

If you found the response to be a useful solution to your question, please mark it as the best answer by clicking “Select as Best” to make it easier for other users to find. Thank you!

Best regards,

Ma. Cassandra | Docusign Community Moderator

"Select as Best" below if you find the answer a valid solution to your issue

Hello ​@Jalal 

I hope you are doing well. 
I want to confirm if you were able to solve your issue by utilizing the suggested solution or if the information provided was useful.
Let me know, and I will gladly help you address the situation as soon as possible.

Ma. Cassandra | Docusign Community Moderator
Mark as “Best Answer” if this helped clarify things.