If you’re using a free subdomain from us, the only TXT records you can add are SPF records, which you can add under the “SPF Records” section of the Control Panel or under the “DNS Records” section of your hosting account on your Client Area.
If you’re using a custom domain, you can also use other DNS providers such as Cloudflare, point your domain to the free hosting’s server IP your domain is on and then add other TXT records, like DKIM and DMARC. The full guide for Cloudflare is here: