For Electrum testnet on Windows OS:
Right-click the Electrum shortcut; In the Properties-Shortcut, edit the Target column by adding the following text at the end:
Although, Ii am using Windows OS. On my laptop, which was window 10, I noticed when I downloaded Electrum on it when I bought it 2 years ago or so, the testnet was created automatically.
I got another laptop running Window 11 which I have just downloaded Electrum on recently is which you can see from above image. The testnet also was created automatically.
And both the Electrum and the testnet Electrum works perfectly.