The primary taxpayer is the individual listed first on the tax return, not necessarily the one has the higher income, or pays more taxes. Keep in mind that the IRS prefers consistency in the spouse naming order of joint filers from year to year, but it’s not the end of the world if the order changes.
When using Credit Karma Tax, the taxpayer who is the Credit Karma Tax account holder must be designated as the primary taxpayer. In other words, you won’t be able use your Credit Karma Tax account and list someone other than you as the primary taxpayer. If you’re filing a joint tax return, it must be prepared and submitted from the appointed primary taxpayer's account.
If you wish to use your spouse or partner as the primary taxpayer, and they do not have an active Credit Karma Tax account, they can, they can sign-up here.