From within Settings, select the "In Meeting (Basic)" option and scroll until you see Co-Host listed as an option; toggle it to on as seen below
Now that Co-hosting is enabled for your meetings, you will be able to assign a co-host after starting a meeting. You will still need to be the original host to being the meeting.
To assign a co-host while the meeting in the session, first the co-host must join the meeting as a participant as usual. Open the Participants list once they have joined the meeting.