JSFiddle - React, Tailwind, and code Playground

HTML

<div class="chat-input-box">
    <div class="w-85">
        <input type="text" class="chat-input-text" name="message">    
    </div>
    <div class="w-15 button-cont">
        <input type="submit" value="Отправить" class="chat-submit-button">
    </div>
    <div class="clr"></div>
</div>

CSS

.w-85, .w-15 {
    box-sizing: border-box;
    float: left;
}
.w-85 {width: 85%;}
.w-15 {width: 15%;}
.clr {clear: both;}

.chat-input-box {
    background-color: #EFEFEF;
    width: 100%;
    padding: 10px;
    box-sizing: border-box;
}
.chat-input-text, .chat-submit-button {
    width: 100%;
}

.button-cont {
    padding-left: 10px;
}