The current web vaults do not have an autofill button, although one is “on its way” per the UI refresh conversation. Today, one clicks on the vault entry itself.
But, that only works on the suggestions. For search results, clicking becomes “view”. There is a point behind this behavior. Auto-fill is a an important safety feature. By requiring the URL to match, it becomes much harder to fall victim to look-alike web sites because the break in work-flow causes one to pause and ask questions.
The true answer, as @grb says, is that the vault entry needs to have both websites/URIs listed on it, not just one. To do this, edit the vault entry scroll down below the “URL 1” field and there is a button to “Add URI”. This will result in auto-fill matching both websites.