You can use querySelector to target the paragraph with the b id.
Then use nextElementSibling to get the div right after and finally innerText to get the expected information.

Optionnally, It may be a good idea to use trim to remove any spaces before or after that text.

const myVariable = document.querySelector('#b').nextElementSibling.innerText.trim()

console.log(myVariable)

.hidden{
  display: none;
}

<div class="hidden">
  <p id="a">66 message</p>
  <p id="b"></p>
  <div data-wrapper="true" style="font-family:'Segoe UI','Helvetica Neue',sans-serif; font-size:9pt">
    <div>test</div>
  </div>
  <p></p>
  <p id="c">66666</p>
</div>

I am wondering why you did not add an id to the div itself.

But then we can get the b id with id selector

const myVariable = document.getElementById('b').nextElementSibling.innerText // getting the content of `test` from the div

console.log(myVariable);

document.getElementById('b').innerText = myVariable // storing in the `b` id

.hidden{
  display: none;
}

<div class="hidden">
  <p id="a">66 message</p>
  <p id="b"></p>
  <div data-wrapper="true" style="font-family:'Segoe UI','Helvetica Neue',sans-serif; font-size:9pt">
    <div>test</div>
  </div>
  <p></p>
  <p id="c">66666</p>
</div>