Startups: Companies that are in their initial stages of business and typically developing a business model and seeking financing.
Microsoft requires all Microsoft for Startups applicants to have a LinkedIn account. If accepted, they use your LinkedIn profile to help you grow and activate your network. Create a LinkedIn account
when you move to Stage 3, you get new benefits starting from the date you reach Stage 3.
For the login issue, I would recommend you contact Azure support or the Founders Hub program support team.
See: https://learn.microsoft.com/en-us/microsoft-for-startups/application/faq?source=recommendations
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Regards,
Abiola