Worth signing up with your corporate email, as the email address associated to your account will be automatically assigned as the owner of the new tenant.
When you login with GitHub, an authorization will be requested to associate the account with the primary email address configured for it.
Upon successful login, you will receive a one-time request to verify the email address, as in the screenshot below.
A request to validate your Azure AD email
To allow Arnica's successful authentication of users in your Azure Active Directory organization, Arnica's application needs to be authorized as the follow up step, as in the screenshot below.
Authorize Arnica's app in your Azure AD
An administrator of your Azure Active Directory needs to approve this authorization. Hence, if you do not have this permission, try other authentication methods first.
When logging into Arnica using Google, you will be forwarded to Google's authentication system. Here you will need to select an email to use within Arnica.