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 by making changes to the secure message invitation for the patient. When you're done, to save the template, click on the "Templates" menu and select "Save Template".
If you want to use a template you previously saved, simply choose from dropdown list of previously saved templates located in the "Templates" menu and it will update the subject line, body, completion notification, and even the forms you included when you saved 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").