@Fritz Salar Thanks for reaching out. This is a known UI bug in the azure portal, and our product team has already fixed the issue. Currently, we are working on the roll out of the fix and unfortunately, there is no ETA available at the moment (will take few days to roll out to all regions). Sorry for the inconvenience caused by this issue.
As a workaround, please use CreateEventSubscription blade without any params and then select your desired topic: Create Event Subscription - Microsoft Azure As you are specifically making the selection here, create button should get enabled.
I hope this helps and let us know if any questions.
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