In the TXT line, enter your domain name or your sub-domain and your SPF record. Do not forget to add a dot (.) at the end of your domain name.
Click the Add button.
On some servers we had to add a backslash ( \ ) before each space otherwise the spf entry would not work.
So if you get a weird result like "v=spf1" "a" "mx" "~all" with the standard spf record, please try with v=spf1\ a\ mx\ ~all