The default way is using Microsoft Account to connect VSTS. But seems you don't have enough permission with your MS account on the project. You may need to ask the Project Administrator or the one invited you to give you enough permission.
If you want to login VSTS with your company account.
Sign up for Visual Studio Team Services with a work or school account
Control access to Visual Studio Team Services the same way you do with
Microsoft services like Office 365 and Azure. When you sign up with a
work or school account, your Visual Studio Team Services account is
connected to your organization's directory (tenant) in Azure Active
Directory (Azure AD). You can then use the same username and password
as other Microsoft services, like @fabrikam.com. Azure AD helps you
enforce policies around accessing your team's critical resources and
key assets.
More details, please refer link from MSDN: Sign up for Visual Studio Team Services: Git & Agile for DevOps, continuous integration, & continuous delivery