Put the incoming call on Hold.

The call will now show on Hold.

Enter the number you want to dial and click the telephone icon.

You now see both your calls. The new active call in light grey, and your on hold call.

Press the Transfer button to transfer the call.

Hover over the the call you want to transfer on the left. You will see Click to transfer appear. Left click to complete the transfer.

Both calls end on your Zopier but are transferred.