Just noticed that the thread has been closed. This has been an ongoing issue since 2021. I still haven't been able to add it.
What old thread are you referring to? Using Citi Cards as the financial institution name, I've been able to add accounts and download transactions without problems for years. You might want to give that a try, using Tools / Add Account, even if you already have created a new offline register within Quicken. Once the Add Account process has found the new account at the bank, LINK it the already existing register or allow creation of a new account register.
To use the Citi Cards Add Account setup link there is something you will first need to do in your Citi.com online account before you can complete the account setup process in Quicken: