Thanks for all the responses. I've managed to solve it. As per explanation by @AnandVyas, when launching SMC cached credentials are not available yet, so when i go register tables, it asks for a two prompts, first is on the SASApp which is a domain account (Default Auth) that launces the workspace, followed by the prompt which asks for the username and password for the SQL Server (DB Auth). I didn't notice that prompts initially so I was actually trying to key in the SQL server account when it as still asking for the domain account.
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