Open local Powershell profile in Notepad:

notepad $PROFILE.CurrentUserCurrentHost

Add following line in your profile file, save, close it.

$PSDefaultParameterValues['Invoke-Sqlcmd:Encrypt'] = 'Optional'

Note: Create Powershell profile if not already existing:

if (-not (Test-Path $PROFILE.CurrentUserCurrentHost)) {
    New-Item -Path $PROFILE.CurrentUserCurrentHost -ItemType File -Force
}