If you need to set an SPF record for a domain on Siteground Web Hosting you can do this via the Control Panel on your account.
Setting up an SPF Record on Siteground Web Hosting
1. Log into the main Control Panel on your account
2. Go to Email / Email Authentication
3. Select Enable SPF record for this account – this will add a record for each domain on the account
4. This can take up to 72 hours to take effect