Hi @Rudyk, Zhanna B,
Thanks for reaching out to us. We are very pleased to support you.
Unfortunately, SharePoint does not support self-sign up.
Here, we have listed the self-service registration programs supported in M365, but SharePoint is not included among them.
Also, in this article it is clearly stated that self-service sign-up user flows are not available for SharePoint.
This is actually the default behavior of SharePoint at this moment.
When we access SharePoint, we need to log in to verify that we have permission to access the content. Therefore, SharePoint requires us to register users in advance and assign appropriate permissions to them before they can access SharePoint properly.
While we can't achieve this at the moment, we suggest that you can make suggestions on this SharePoint Feedback portal for self-sign up in SharePoint. With higher feedback like this, MS will take this into serious account and make some modifications accordingly, which will benefit more community members. Your feedback and suggestions are very valuable.
Should you have any concern or queries, please feel free to contact us.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. It will be beneficial to more community members reading here. Your contribution will be highly appreciated.