Hi,
Microsoft certification does not come for free, but you may register for free events that sometimes offer free exam vouchers.
As far as I know Microsoft does not have any certification for digital marketing, but you may also ask the more subject matter experts in the new dedicated Microsoft certificaiton forum over here:
https://trainingsupport.microsoft.com/en-us/mcp/forum/mcp_cert-mcp_paths
Best regards,
Leon
Blog:
https://thesystemcenterblog.com
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