Edit: sorry, the below if for Mac Safari. I didn't realize what thread category I was in.
- Right click on the Safari toolbar to display Customize Toolbar...
- Then drag and drop the Bookmarks Star icon up where you want it in your toolbar.
I don't think you can add a button... but you can click and hold on the URL (website address) for a few seconds and one of the options that will appear is for making it a bookmark.