How can I remove this using CSS or JS?

[AkhilKokani]

This is an image of a textbox when it has been clicked, I don’t want to show the blue colored border when it is being clicked. How can I do this?

Thanks

[Ray.H]

See this page for more info on what you are wanting to do.
CSS outline property

If after reading you still feel you must remove it then provide alternative styling and do this …

Summary

textarea {
    outline: none;
    box-shadow: none;
}

[felgall]

@AkhilKokani

I don’t want to show the blue colored border when it is being clicked

That is effectively the keyboard cursor so people without a mouse can see where they are in the page for when they interact using the keyboard. If you are going to remove it you need to replace it with something more obvious so that you don’t break accessibility.