I'm sure there must be a simple solution to this but I can't seem to figure it out.
I have a site that has SAML authentication. Is there a way to initiate the login from a link or button? i.e login button in the header that brings the user to the IDP page so it can redirect back after login?
It depends if the SAML authentication is SP initiated or IDP initiated. This can be configured either way and you can check the SAML configs to get an idea.
Usually AEM customers I have seen have SP initiated SAML authentication configured, where they just hit the author url or section in the publish which is secured by authentication to trigger the SAML auth.
That's what we have been doing, hitting a page that required authentication but when the authentication is successful the user is redirected back to the page that requires authentication. I.e. a user is on /index.html, clicks on Login in the nav, which is a link to /login.html that required authentication, SAML successful authenticates and brings the user back to login.html when ideally we want to go back to index.html.