If you haven’t done it yet, you will. Setting up many VMs for testing or development will make you happy click past the ISO choice when you click start. Then startup fails because you have no media. Most people cuss, delete the new VM entry, then start again from scratch, going slower this time. You don’t have to.
Choose the Empty CD icon.
Click the down carrot next to the CD image.
Select “Choose Virtual Optical Disk File” then use your host file selector which pops up to select the ISO file you need.
Be sure to check the “Live CD/DVD” box so Virtualbox knows the media should be considered bootable. That’s it! Save your changes and restart your VM. After you finish the install you need to come back here and remove the ISO from the virtual CD drive.
You can also use this same method with a GParted ISO to expand your virtual disk if you run out of room.