The RJ9 cable supplied is standard phone handset wiring and shouldn't be used with a real Mac Plus keyboard - you would kill it. Don't be tempted to power the Arduino compatible board externally, it will make no difference and might kill your Mac.
eBay
2 In 1 OTG Adapter For iPhone 16 15 14 13 12 Pro Plus 8 Pin To USB Cable 3.0 Converter For Mouse Keyboard Camera Card Reader
Aliexpress UK
This adapter will work if the or keyboard your using it with supports this conversio. -up USB ports for other devices.
eBay
Grwibeou USB Bluetooth 5.1 5.0 Dongle Adapter for PC Speaker Wireless Mouse Keyboard Music Audio Bluetooth Receiver Transmitter
Aliexpress UK
PCI Revision: 2.2. USB Compliance: OHCI (USB 1.0A), EHCI (USB 1.0). USB HID Support: V1.1. 1 x Adapter Card. Users may also refer to online installation tutorials for self-installation. Data Transfer: 32-bit, 33MHz.
eBay
HiFing USB IN-OUT MI Cable One In One Out Interface 5 Pin Line Converter PC to Music Keyboard Adapter Cord
Aliexpress UK
Note: Because this product without the chip, there is a major defect of the product. it will pick the keyboard. Due to the different display and different light, the picture may not show the actual color of the item.
eBay
Bluetooth Adapter for Pc Usb Bluetooth 5.3 Dongle Bluetooth 5.0 Receiver for Speaker Mouse Keyboard Music Audio Transmitter
Aliexpress UK
If you have used this adapter or know that your keyboard and mouse have a chip, just shoot it! One end: USB Type A female.
eBay
Bluetooth Adapter for Pc Usb Bluetooth 5.4 5.3 Dongle Receiver for Speaker Mouse Keyboard Music Audio Transmitter
Aliexpress UK
This adapter will work if the Mouse or keyboard your using it with supports this conversion. Free-up USB ports for other devices. Buy Two get One Free!
eBay
KVM Extender 60M 1080P HDMI-compatible Over cat5e/6 Ethernet RJ45 LAN Cable USB-A Keyboard Mouse KVM Video Extend TX RX Adapter
Aliexpress UK
❤【ONLY SUPPORT MOUSE AND KEYBOARD】-- The usb interface only used to connect mouse or keyboard.It can not connected with other devices. ❤【MATCHING PROBLEM 】-- This adapter can't fit all the motherboard of different computer.So it have matching problem Only match generic mouse and keyboard.
eBay
MINI USB Bluetooth 5.1 Dongle Adapter for PC Laptop Mouse Keyboard Tablet Phone Printer Game Speaker Wireless Transmit Receiver
Aliexpress UK
And also you need to connect your mouse or keyboard before you start up your PC.
eBay
Professional MIDI Cable USB IN-OUT PC to Music Electronic Keyboard Adapter Cord Musical Enjoyable Instrument Supplies
Aliexpress UK