I too came here to say that this is already quite easy. We have a menu item on the homepage of our website: “Client Portal” - when onboarding clients I remind them if they ever can’t find a link or forgot to bookmark it, they can access it from our website.

What is your website built on? You would just need to add a button to your website wherever you want it and then add the Portal URL to the button so when people click on it, it takes them straight to the portal

You can already do this, it just has to be set up on the back end of your website. On my site, I have a “Client Portal” tab in the menu that links directly to my branded Double portal.