HTML ISINDEX <isindex> tag is used to provide a single-line text input in a page to query a document. If a user wants to send a query using the input to the server then the server returns the list of many pages matching with the query.
This tag is not supported by HTML5, so you can use form <input> which will do the same work.
<title>HTML isindex Tag</title>
<isindex prompt = "Search your document here"/>
<h2>Example of isindex tag</h2>
this is body