html - how to get custom attribute of option tag in dropdown?

You need to figure out what the selectedIndex is, then getAttribute from that options[] Array.

<select onchange="alert(this.options[this.selectedIndex].getAttribute('isred'));" name="myname" class="myclass"> 
    <option isred="-1" value="hi">click</option>
    <option isred="-5" value="hi">click</option>
</select>​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

jsFiddle DEMO

As a side note:

Don't use inline javascript in your HTML. You want to separate your business logic from your UI. Create a javascript event handlers instead to handle this. (jQuery / Angular / etc)

//Pure Javascript solution, and elegant one check if you really want to leverage the power of javascript.

// Listening to a onchange event by ID attached with the select tag.
document.getElementById("name_your_id").onchange = function(event) {

//event.target.selectedOptions[0] have that option. as this is single selection by dropdown. this will always be 0th index :) 
let get_val = event.target.selectedOptions[0].getAttribute("isred");
console.log("Value from the Attribute: ", get_val)
}

 <select id="name_your_id" name="myname" class="myclass">
    <option isred="423423" value="hi">One</option>
    <option isred="-1" value="hi">Two</option>
 </select>

Use something like this:

document.getElementById("x").onchange = function () {
    console.log(this.options[this.selectedIndex].getAttribute("isred"));
};

in jquery, you can just write:

$("#myname").find(':selected').attr('isred');