Adding a co-signer to your Student Loan Refinance application does not guarantee an interest rate change. For a co-signer to impact the interest rate, it will depend on the co-signer's ability to meet SoFi's eligibility criteria.
To see if your co-signer will impact your interest rate, they will need to be added to the application. While adding a co-signer is not required, it may help you qualify for a loan or a lower interest rate.