IRAs are titled in the name of an individual. If you’d like to make an IRA contribution for your spouse or partner at Betterment, you’ll need to have them open a Betterment IRA in their own Betterment account. You can then fund that account from your shared bank account, if applicable.
Once you do this, you can contact Support to “household” the accounts together, so that you benefit from each other’s balances for the purposes of fee calculations. Joint accounts automatically household the associated personal accounts.