Prepare Canvas

index.html

<script src="https://d.line-scdn.net/liff/1.0/sdk.js" /></script>
<script src="https://cdn.jsdelivr.net/npm/signature_pad@2.3.2/dist/signature_pad.min.js"></script>
  <script>
    const canvas = document.querySelector('#canvas');
    const signaturePad = new SignaturePad(canvas, {
      backgroundColor: 'rgb(238,238,238)',
    });

    $(window).on('load', function(){
      canvas.setAttribute('width', $('.container').width());
      canvas.setAttribute('height', window.innerHeight - $('#btn').outerHeight() - 10);
      signaturePad.clear();
	  // Initializes a LIFF app
      liff.init(function (data) {});
    });
  </script>