Indexed Search
1.2. User manual
1.2.1. Adding the search plugin to a page
That is really easy:
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Create a page called "Search" or something like that. This is where the search box will appear.
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Then create a new content element on that page. From the Web>Page module you can do it like this:

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Then select some plugin-type if you can. It doesn't matter if it's a guestbook or forum. Or if no plugins are available, just select a "Regular text element" as in the top of the page.

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Then make sure "Insert plugin" is selected (if not, select it and save the element, then you'll see the form below), enter a title and select the "Plugin" type to be "Indexed search":

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Then select the root page of your website as the "Starting point" of the plugin content element:

And that's it. Your frontend should now look like this:

The styles are most likely different from this, but that is controlled by the developer having administration access to the system.