Javascript input code

Here is html code.

    <div data-block="true" data-offset-key="7cq06-0-0" class="_209g _2vxa" data-reactid=".c.1:3.0.$right.0.0.0.0.1.0.0.1.0.0.$7cq06">
<span data-offset-key="7cq06-0-0" 
data-reactid=".c.1:3.0.$right.0.0.0.0.1.0.0.1.0.0.$7cq06.0:$7cq06-0-0"><br 
data-reactid=".c.1:3.0.$right.0.0.0.0.1.0.0.1.0.0.$7cq06.0:$7cq06-0-0.0"></span>
</div>

I want to enter a input automaticaly by javascript code. After enter "MESSAGE HERE !!!" the html code changes like this(I entered by hand) :

    <div data-block="true" data-offset-key="2s0n6-0-0" class="_209g _2vxa" data-reactid=".h.1:3.0.$right.0.0.0.0.1.0.0.1.0.0.$2s0n6">
<span data-offset-key="2s0n6-0-0" 
data-reactid=".h.1:3.0.$right.0.0.0.0.1.0.0.1.0.0.$2s0n6.0:$2s0n6-0-0"><span 
data-reactid=".h.1:3.0.$right.0.0.0.0.1.0.0.1.0.0.$2s0n6.0:$2s0n6-0-0.0">MESSAGE HERE !!!</span></span></div>

What is the javascript code that I am looking for ?