you can use button in custom login page and set the on click action as open Link then pass the azure link like this.
You can use a button in that custom login page and set the on click action as open Link.
Then set that SSO link to the button.
If I understand you correct you have written a custom login page using html? So login.html? This way of providing a login page only works when you don't need any mendix logic before the user logs in. In the case of using mendix SSO you need mendix logic before the user logs in. Mendix needs to retrieve the SSO settings before you can redirect the user to the identity provider. So you have have to let a anonymous user land on a mendix page with your SSO button.
You can always customize your mendix page btw, anything you can do in login.html you can recreate in a mendix page.
I believe there’s a learning path that can help you out: Mendix Academy - 3.4 Custom Login Page, here, they are building a custom log in page, with custom front-end widgets.
in this custom login page, you can add the button that redirects to your /SSO/
I hope this helps.