I believe you can do this through the Email Resource file under Branding. Try this:
- Login to your developer sandbox at demo.docusign.net
- Click your profile icon in top right, then go to
Preferences -> Branding
. - Select the branding profile you are currently using, or create a new one.
- On a given brand, select the
Resources
tab, then select the Email Resource file. Search for this string in your Email resource file:
<data name="RecipientEnvelopeComplete_Subject">Completed: [[SubjectInput]]</data>
Try changing that to the new subject text you'd like and that should do the trick.