Try adding element to the iframe document body:

function Addcompntn() {
    var framedeki = document.getElementById('if1').contentWindow.document;   
    var div = document.createElement("div");
    div.id = 'innerdiv';
    div.style.color = 'white';    
    framedeki.getElementById('example').append(div);
}

I want to add input in a div content javascript but it doesn’t add

HTML:

<button onclick="Addcompntn();" class="btn btn-success">Add</button>
<iframe class="embed-responsive-item" src="index.html" allowfullscreen id="if1"></iframe>

Script:

function Addcompntn() {
    var framedeki = document.getElementById('if1').contentWindow.document; 
    $('example').append('<div id="innerdiv" style="color:white;"></div>');
}

index.html

<div id="example" style="z-index:2"> //dynamic data will come here </div>