<form method="post" name="testform" action="#">

    <a href="#modal"> <!-- when the input textbox was clicked, modal will pop up -->
        <input readonly type="text" name="txt1" placeholder="inputTest">
    </a>

    <div class="modalwrapper" id="modal">   <!-- modal -->
            <div class="modalcontainer">
                <div class="modalcol1">
                    <label>Test 1</label>
                    <input type="checkbox" name="test_modal[]" value="1">
                    <div class="clear"></div>
                    <label>Test 2</label>
                    <input type="checkbox" name="test_modal[]" value="2">
                    <div class="clear"></div>
                    <label>Test 3</label>
                    <input type="checkbox" name="test_modal[]" value="3">
                    <div class="clear"></div>
                    <label>Test 4</label>
                    <input type="checkbox" name="test_modal[]" value="4">
                    <div class="clear"></div>
                    <label>Test 5</label>
                    <input type="checkbox" name="test_modal[]" value="5">
                    <div class="clear"></div>

                    <div class="savebutton">
                        <button class="btn1" type="button" value="Submit">Submit</button>
                    </div>
                </div> <!-- close modalcol1 -->
            </div> <!-- close modalcontainer -->
        <div class="overlay"></div>
    </div> <!-- close modalwrapper -->
</form>

/* modal layout */
    .modalwrapper {
        top: 0;
        left: 0;
        opacity: 0;
        position: absolute;
        visibility: hidden;
        box-shadow: 0 3px 7px rgba(0,0,0,.25);
        box-sizing: border-box;
        transition: all .4s ease-in-out;
        -webkit-transition: all .4s ease-in-out;
        -moz-transition: all .4s ease-in-out;
    }

    .modalwrapper:target {
        opacity: 1;
        visibility: visible
    }

    .overlay {
        background-color: #000;
        background: rgba(0,0,0,.8);
        height: 100%;
        left: 0;
        position: fixed;
        top: 0;
        width: 100%;
        z-index: 1;
    }

    .modalcontainer {
        display: table;
        background-color: #777;
        position: relative;
        z-index: 100;
        color: #fff;
        padding: 5px;
    }

    .modalcol1 { display: table-cell; }

    .clear { clear: both; }

    .modalwrapper input[type=checkbox] {
        float: right;
        margin-right: 20px;
    }

    .savebutton input[type=submit] {
        float: right;
        background-color: maroon;
        color: #fff;
        border: none;
        padding: 5px 10px;
        margin-top: 5px;
        margin-right: 20px;
    }
    /* modal layout ends here */