How to 'Grant Permissions' Using Azure Active Directory PowerShell V2
This button is effectively doing admin consent. This will consent for all users in the tenant. For your case, you can force consent in the SPA rather than in PowerShell if you want to avoid the Azure Portal.
To do this, your SPA should append on the auth request either &prompt=consent or &prompt=admin_consent. The former should be applied each time a new user signs in for the first time, whereas the latter you could do one time (sign in w/ an admin account) and it would consent for all users.
Checkout understanding Admin and User Consent.