When creating a connection/connection field you have a few advanced rules that enable you to determine how this connection will work.
One of these rules is Autofill. By enabling autofill you'll be able to 'connect' the fields in the origin pipe to the fields in the pipe/database's start form you're connecting to:
To enable the connection's autofill, check the box next to the option above. Once you do, this window will pop-up will show up:
You'll see the pipe/database's start form and, when you click the "+" in the dropdown menu under the field's name you'll be able to choose from the fields of the origin pipe. It's important to note that you'll only be able to connect fields that have the same type (number > number, text > text, etc.).
Ps. The autofill feature is currently active using the connection feature (both to pipe and database) and using the connection field (between pipes or a pipe and a database).