<input type="checkbox" id="checkbox">
<input type="text" id="textbox" disabled>

const checkbox = document.getElementById('checkbox');
const textbox = document.getElementById('textbox');

checkbox.addEventListener('change', function() {
    if (this.checked) {
        textbox.disabled = false;
    } else {
        textbox.disabled = true;
    }
});

External CSS

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External JavaScript

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