CSS: checkbox not checked when spacebar is pressed

I have the simple form below (and here: https://codepen.io/anon/pen/QxKrex). The checkbox works when you click on it, but I would like that when you have the focus on it (after a "tab" from the textfield) and press spacebar, it would be marked as well. However, it doesn't seem to work. Any ideas?

index.html

.labelCheckbox {
        display: contents;
    }
    
    .checkbox label {
        float: left;
        padding-left: 0px !important;
        font-size: initial;
    }
    
    input[type=checkbox] {
        opacity: 0;
    }
    
    input[type="checkbox"]{
        display: none;
    }
    
    input[type="checkbox"] + label::before{
        background-color: white;
        border: 1px solid #135BCF;
        content: '';
        display: inline-block;
        height: 22px;
        width: 22px;
        text-align: center;
        margin: 0 5px -2px 0;
        overflow: hidden;
        top: 3px;
        position: relative;
        float: initial;
    }
    
    input[type="checkbox"]:checked + label::before{
        content: '\2713';
    }

<form action="/send.php" method="post" name="myForm">
      <label for="fname"></label>
      <input type="text" id="fname" name="fname" placeholder="Name" required>
        <input type="checkbox" id="checkbox1">
        <label for="checkbox1" tabindex="0;"></label>
      <label class="labelCheckbox">&nbsp&nbspI agree</label>
      <button class="modalButton" type="submit" value="submit" id="submit2">Submit</button>
    </form>