Password protecting ad-hoc networks has been removed for some while. It seems creating ad-hoc networks via System Preferences>Network is deprecated as Apple's macOS manual explicitly notes to "enable
legacy networks checkbox via advanced options" when setting up an ad-hoc network (on recent macOS versions.)
Assuming your switch is connected to Mac mini's ethernet port, you can try this:
- In System Preferences>Sharing choose "Internetsharing."
- Select destination "Ethernet" from the dropdown.
- Select sources via checkbox, e.g., "WLAN."
- Set your WLAN options via button below including SSID, channel, Auth-protocol and password.
This will relay all WLAN traffic to ethernet network.