use textarea.value!

call/static/call/xpubcharacters.js

@@ -48,7 +48,8 @@ function updateCaption () {
   // grammar.forEach(key => {
   //   caption += (caption ? " " : "") + terms[key][cParts[key]];
   // })
-  document.querySelector("#caption").innerHTML = caption;
+  // document.querySelector("#caption").innerHTML = caption;
+  document.querySelector("#caption").value = caption;
 }
 
 async function start () {
@@ -104,7 +105,7 @@ function send (canvas, form) {
       const formdata = new FormData();
       const filename = new Date().toISOString() + ".png";
       formdata.append("image", blob, filename);
-      formdata.append("caption", document.querySelector("#caption").textContent);
+      formdata.append("caption", document.querySelector("#caption").value);
       formdata.append("csrfmiddlewaretoken", form.csrfmiddlewaretoken.value);
       fetch(form.action, { method:"POST", body: formdata })
         .then(() => {