How can I use rEFInd boot manager without an enter key?
You can try using one of the following:

rEFInd does have the ability to work will a mouse. If this does not work automatically, then you may have to edit the refind.conf file. This file can be found in the EFI/BOOT folder of the EFI volume labeled ElTorito on the USB.
By default, you can make your selection by pressing a number key. So for example, if the first row on the displayed rEFInd menu has three icons, the you can make a selection by pressing either the 1, 2 or 3 key. In fact, you can do this before the menu is even displayed.
The return key has the ASCII code of 13, which is a control M. So you can substitute pressing the control+M key combination for pressing the return key.