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I'm sorry that I was not clear. I will try to elaborate.
I'm trying to add a txt record to my domain lsacam.com. I do this by going to My Domains > Domain Management > Add. In the Host field I add a value called "_acme-challenge.cam.lsacam.com" and in the TXT Value field I add a value called "YxnxHMWxx0564IycsxLT44xb307zPoz2W3sYo58v1tw" which is a challenge associated with using Lets Encrypt as a CA (domain validation method, see https://serverfault.com/questions/879647/renew-domains-using-certbot-and-using-dns-challenge). Note that value above is generated by the certbot tool and will be different from run to run.
I poll to see when the record gets updated by the following using the linux command "dig -t txt cam.lsacam.com". But even after days it does not get added.
However if I use "@" instead of "_acme-challenge.cam.lsacam.com" as the Host value then I see the TXT record get added after the TTL period. However Lets Encrypt for what ever reason fails with that. It must need the "_acme-challenge.cam.lsacam.com" Host name and indeed this is how I got it to work before (about 3 months ago).