Hi Björn Peter,
Transferring Azure Sponsorship (MS-AZR-0036P) credits to an existing subscription is not a self-service action and requires assistance from the Microsoft for Startups support team. To initiate this process, please submit a support request or or create a support ticket with the billing team here and clearly state that the issue relates to an "entitlement transfer." Be sure to include details of both your existing subscription and the newly created subscription involved in the transfer.
Even if you do not have a support plan, the Microsoft for Startups team can assist with entitlement transfer requests.
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