Migration Assistant Time Machine Disc Volume Issue

Purchased a used iMac, running Ventura 13.5. Trying to move my data from my old iMac to the new one via my Time Machine backup and the Migration assistant. The new iMac has a 128GB SSD drive as the startup disc and a 2TB Fusion Drive. The Migration assistant only gives me the option the restore to the 128GB drive and there is not enough space. How to I make the 2TB drive available for the migration assistant to restore the user data a file to? I believe that I want to keep the 128GB SSD drive as the startup disc for speed.