Enter TXT Record
In order to create a TXT record, log into your account and then go to Domains -> My domains in the menu at the left and choose a domain.

Go to DNS settings and click on the Edit button next to your domain.

In the pop-up window go to the TXT section and enter the desired data in the TXT record field. If necessary further lines can be added. Don`t forget to save your entries.

Please note
It needs time for changes in the DNS to take effect.
In case a single TXT record is longer than 255 characters you have to split it into two parts (like "1stpart" "2ndpart") to add it into a single line. This is because Strings in DNS records have a maximum of 255 characters according to RFC.
By splitting the record with double quotes this limitation can be avoided.