You can transfer a call in two ways:
Using Quick Connects:
When connected to the customer, click Quick Connects.
Choose a quick connect from the list and then click Call.
Using the Number Pad:
Click Number pad.
Type the external number you wish to transfer to and then click Call.
For the official AWS guide on this lesson you can visit https://docs.aws.amazon.com/connect/latest/adminguide/transfers.html