Flex mini switch has 5 ports 1st port is PoE IN support 802.3af and it is powered by a 5V, 1A Power Adapter available inside the box which is USB type C.
Adopting a flex mini switch is different from all other UniFi devices as it doesn't support SSH so you have to adopt them via DHCP option 43 or DNS method or local discovery L2 method.
Local controller L2 discovery method (Recommended):
You can have your local UniFi controller running in this method and make sure the switch is connected to Network and powered.
Go to settings >> controller >> enable Make controller discoverable on L2 network and save the changes.
Go to the devices, tab Flex mini should discover as Pending adoption.
After adopting into local UniFi controller you can migrate a site to HostiFi and move the devices to specific sites for a site migration you can follow this guide https://support.hostifi.com/en/articles/3440680-migrate-your-unifi-servers-to-hostifi-with-the-import-export-wizard
After migrating a site to HostiFi you can move it click on switch >> go to config >> manage device >> under forget device there is an option to move the devices, It will be connected to your site.
DHCP option 43 :
Once you powered the switch and connected to the LAN network, you will have to enable DHCP option 43 on your Router LAN network.
Follow this guide to enable DHCP option 43 https://help.ui.com/hc/en-us/articles/204909754-UniFi-Layer-3-Adoption-for-Remote-UniFi-Network-Applications#7
You can enable DHCP option 43 on third party routers such as Pfsense, Mikrotik, Palo Alto etc following the above guide mentioned.
If you have USG as your router Go to settings >> Network >> Edit LAN network >> DHCP UniFi Controller put the controller IP address and save the changes.
Once you applied the changes go to the Devices tab and UniFi Flex Mini will show as Pending adoption.
You can adopt it from the controller by clicking to switch and Hit Adopt.