To transfer an active call, the best approach is to use the Quick connects feature. 

Cold or Blind Transfer

To initiate a cold or blind transfer complete the steps above but after clicking Call, you can then immediately click End call to drop off the call and the original caller will still be transferred to the quick connect call.

Demonstration Video

Transfer and Conference.mp4

For the official AWS guide on this lesson you can visit https://docs.aws.amazon.com/connect/latest/adminguide/transfers.html