This is how I did it with Powershell:
$extUid,$extVersion = 'vscjava.vscode-java-pack','0.30.4'
$extPubl,$extName = $extUid -split '\.'
$extUrl = "https://marketplace.visualstudio.com/_apis/public/gallery/publishers/$extPubl/vsextensions/$extName/$extVersion/vspackage"
Invoke-RestMethod $extUrl -OutFile "~/Downloads/$extUid.vsix"
code --install-extension "/home/user/Downloads/vscjava.vscode-java-pack.vsix"
# Installing extensions...
# Extension 'vscjava.vscode-java-pack.vsix' was successfully installed.
Url construction described here: https://stackoverflow.com/a/79565372/20340543