Solution
$userName = 'test-domain\test-login'
$password = 'test-password'
$pwdSecureString = ConvertTo-SecureString -Force -AsPlainText $password
$credential = New-Object -TypeName System.Management.Automation.PSCredential `
-ArgumentList $userName, $pwdSecureString
For Build Machines
In the previous code replace user name and password values by secret ("hidden from logs") environment variables of your build-machine
Test results by
'# Results'
$credential.GetNetworkCredential().Domain
$credential.GetNetworkCredential().UserName
$credential.GetNetworkCredential().Password
and you'll see
# Results
test-domain
test-login
test-password