Hi Kanagaraj Neelamegan,
SharePoint online did not expose SSOM so we think there is no oob functionality to get this xoml file. We found that we can export this file from SharePoint Designer:

For more details, please refer to : https://www.linkedin.com/pulse/export-import-sharepoint-designer-workflow-vipul-jain/
By tracing the request via Fiddler, we find that the behind request is :

So we can simulate this process :
$Username='abc@xxx.onmicrosoft.com'
$Password = 'xxxxx'
#region Credentials
[SecureString]$SecurePass = ConvertTo-SecureString $Password -AsPlainText -Force
[System.Management.Automation.PSCredential]$PSCredentials = New-Object System.Management.Automation.PSCredential($Username, $SecurePass)
#endregion Credentials
#connect to site
$targetSiteUrl='https://xxx.sharepoint.com/sites/sbdev'
Connect-PnPOnline -Url $targetSiteUrl -Credentials $PSCredentials
# get cookie
$ctx = Get-PnPContext
$creds = $ctx.Credentials
$authenticationCookies=$creds.GetAuthenticationCookie($targetSiteUrl,$true)
# Get xoml
$url="https://xxx.sharepoint.com/sites/sbdev/_vti_bin/_vti_aut/author.dll"
$request = [System.Net.WebRequest]::Create($url)
$Request.Method = 'POST';
$Request.ContentType = "application/x-www-form-urlencoded"
$Request.Headers.Add("Cookie", $authenticationCookies)
$Request.Headers.Add("X-Vermeer-Content-Type", "application/x-www-form-urlencoded")
$Request.Headers.Add("Accept-Language", "en-us, en;q=0.1");
$postData = "method=get+document:15.0.0.4455&service_name=/sites/sbdev&document_name=_catalogs/wfpub/Collect+Signatures+-+SharePoint+2010/Signatures_1033.xoml&old_theme_html=false&force=true&get_option=none&doc_version=&timeout=0&expandWebPartPages=true";
$Body = [byte[]][char[]]$postData;
$Stream = $Request.GetRequestStream();
$Stream.Write($Body, 0, $Body.Length);
$response = $request.GetResponse()
$requestStream = $response.GetResponseStream()
$readStream = New-Object System.IO.StreamReader $requestStream
$data=$readStream.ReadToEnd()
$data
We can get the xoml file from above result.
This “SharePoint Server 2010” Forum will be migrating to a new home on Microsoft Q&A,
please refer to this sticky
post for more details.
Best Regards,
Baker Kong
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