Hello
It is not possible to post anonymously.
The reason you are seeing the "anonymous" is because those are migrated threads from the "Answers Community". Microsoft used to have the Answer Community for home users, and the Learn Q&A platform you are on now used to be for commercial users. Answers Community got migrated to Learn Q&A earlier this year. A lot of users' profiles turned anonymous after the migration, unless they had elevated contributor status, such as moderators, independent advisors, etc.
You can always click on your profile icon on the upper right, click on Settings. And then change your display name there.