Hi German Feijoo,
Thank you for reaching out to the Microsoft Q&A platform.
You can request delegated admin privileges from the client. This will allow you to manage their licenses and subscriptions on their behalf. The client’s admin can grant these privileges through the Microsoft 365 admin center.
https://learn.microsoft.com/en-us/partner-center/customers/gdap-introduction
The client’s tenant admin can directly purchase the VDI Enterprise VDA licenses through their Microsoft 365 admin center They can either grant you the necessary permissions to make the purchase or handle the purchase themself.
Please find the below documentation for your reference:
https://learn.microsoft.com/en-us/azure/virtual-desktop/licensing
I hope this information is helpful. Please feel free to reach out if you have any further questions.
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Thank You.