How to add static text inside an input form

HTML

<label for="subdomain">Subdomain:</label>
<input type="text" placeholder="ExampleDomain" id="subdomain" />
<input type="text" id="subdomaintwo" value=".domain.com" disabled/>

CSS

input[type="text"]#subdomaintwo{
    -webkit-appearance: none!important;
    color: red;
    text-align: right;
    width: 75px;
    border: 1px solid gray;
    border-left: 0px;
    margin: 0 0 0 -7px;
    background: white;
}
input[type="text"]#subdomain{
    -webkit-appearance: none!important;
    border: 1px solid gray;
    border-right: 0px;
    outline: none;
}

JS Fiddle for this

You can achieve this with the following approach:

• place the <input> in a <label> with position:relative
• give the <label> an ::after pseudo-element with position:absolute
• set box-sizing of the <input> to border-box
• give the <input> a padding-right equal to the width of the ::after pseudo-element

Working Example:

label, input {
    position: relative;
    display: block;
    padding-right: 76px;
    width: 174px;
    box-sizing: border-box;
}

label::after {
    content: '.' attr(data-domain);
    position: absolute;
    top: 4px;
    left: 96px;
    font-family: arial, helvetica, sans-serif;
    font-size: 12px;
    display: block;
    color: rgba(0, 0, 0, 0.6);
    font-weight: bold;
}

<label data-domain="domain.com">
<input type="text" placeholder="exampledomain" />
<label>