InstallLetsEncryptCert.ps1

If (-NOT ([Security.Principal.WindowsPrincipal][Security.Principal.WindowsIdentity]::GetCurrent()).IsInRole([Security.Principal.WindowsBuiltInRole] "Administrator")) {
    Write-Error "Need to run as an administrator"
    return -1
}

$source = "https://letsencrypt.org/certs/lets-encrypt-x3-cross-signed.pem"
$file = $source.Split("/")[-1]
$out = Join-Path "$($env:USERPROFILE)\Downloads" -ChildPath $file

Invoke-WebRequest -Uri $source -OutFile $out

$args = @(
    "-trustcacerts"
    "-keystore"
    '"' + $($env:JAVA_HOME) + '\lib\security\cacerts"'
    "-storepass"
    "changeit"
    "-noprompt"
    "-importcert"
    "-file"
    $out
)

$process = Start-Process -FilePath $env:JAVA_HOME\bin\keytool.exe -ArgumentList $args -Wait -PassThru -NoNewWindow

This assumes that the JAVA_HOME environment variable is set correctly and that keytools.exe is found under it's \bin directory