When you are logged in to your account, go to the domain dashboard for which you want to add the record.
In the Hostname field, if you are setting up the record for the root domain, e.g. mydomain.com, you can leave it blank. It's usually left blank, and once the record is saved the field will show @.
If you are setting up the record for a subdomain, e.g. mail.mydomain.com, then you would enter "mail" in that field.
In the Type drop-down list, select TXT. (Even if you are creating an SPF record.)
Add the Value, which could be an alphanumeric code, a word, or combination phrase.
Leave the TTL as is at 3600.
Click the blue add button, then click Save DNS Settings.