This assumes that you have already logged into you Plesk control panel.

1. Click domains on the left panel under the heading ‘ hosting services ‘
   
   
   
   
2. Click on ‘open in control panel and then click on website & Domains tab.
   
   
   
3. Click on DNS Settings then add record.
   
   
   
4. Select TXT from the drop down list.
   
   
5. Add your domain authentication records into the fields and click ok.
6. To apply the SPF record click the update button.