Select the From (BSC or Matic) at top, the To (FTM) at bottom. Select USDC as the token, and Approve USDC. Now click Swap to send funds from BSC or Matic to the FTM chain.
You can add USDC to your Assets using the USDC contract: 0x04068da6c83afcfa0e13ba15a6696662335d5b75