Run Scripts on websites using this simple technique

What are bookmarklets?

A bookmarklet is a bookmark stored in a web browser that contains JavaScript commands that add new features to the browser.

Developing a bookmarklet is simple and easy to use.

They can even make a post request with the fetch API.

Use javascript in href attribute of <a> tags in HTML.
An example would be

<a href="javascript: alert('hello world');">click me</a>

Make that in HTML and drag it into your bookmarks bar, you have just added a bookmarklet to your browser. Isn't it simple?

All the javascript in your bookmarklet has to be minified.

<a href="javascript: fetch('https://api.website.com/endpoint').then((data) => { alert('data'); })"></a>

Bookmarklet can be used to modify(or filter) the content of a website.
Below is an example:

<a href="javascript: document.body.innerHTML = '<h1>yeah!</h1>' })"></a>

The main reason you should consider developing or using a bookmarklet is for productivity.

There are many ways to install a bookmarklet. I will show one of them which is probably the simplest one.

Search for bookmarklets you want to install on the internet.
Drag the bookmarklet from the page to your Bookmarks Toolbar. It should show on the toolbar now.
If the Bookmarks Toolbar is not visible then you first have to show the Bookmarks Toolbar by right-clicking on an empty section of the Tab Strip and checking the Bookmarks Toolbar in the pop-up menu. Now repeat the above step.