1. In the dashboard for your domain, enter the Hostname. This will often be left blank, but if you're entering an SPF record (which is still in TXT format), it will likely have some combination of letters, numbers, underscores, etc. If left blank the field will display @ once you save the new TXT record.
2. Select TXT from the Type drop-down list.
3. Add the Value, which could be an alphanumeric code, a word, or something more complicated. (Typically you get this unique code from your account for the service you're setting up, e.g. G Suite, Microsoft Office 365, Zoho Mail, etc.)
4. Leave the TTL at 3600.
5. Click the blue add button. The TXT record will now show in the domain's list of DNS records.
6. Click Save DNS Settings.