Yes, you can do this with templates. To edit your templates, navigate to the Patients tab in the Ocean portal, select a test patient, and click "Email".
The default template will appear and you can edit it to your liking. When you're done, to save the template, just click on the template menu after you make changes to a secure message invitation for a patient.
If you want to use a template you previously saved, simply choose from your listed templates and it will update the subject line, body, completion notification, and even the forms you included in the template.
If you want to delete this template, select "Delete Template...". Otherwise, you can save this template as the new default ("Save as Default") or as a new template ("Save Template").