You can apply for an Add-on card while applying online for your SBI Card.
Also, you can apply for an Add-on card by logging to your SBI Credit Card online account in 3 simple steps:
Login to your account on website or mobile app
Go to Services tab > Add-on Card
Fill the application form and submit the KYC documents
Your Add-on Card will be issued to you after necessary verification of the submitted KYC documents