how to make child in flex go full width?

I've started using flex and I love how it works.

I'm using flex layout and I want the btn-group class inside dashboard-body to go full width of the parent. Right now its width is equal to the width of both buttons together.

What I want to achieve here is adding space between both buttons also I don't want to use margin right or left. I want the btn-group div to have full width of its parent dashboard-body so I can use the flex property align-content: space-between to have enough space between both buttons.

Is there any better way other than adding margin/padding around buttons to do the same?

Thanks.

Here is the code:

.dashboard-body {
    display: flex;
    flex-wrap: wrap;
    align-items: center;
    justify-content: center;
    flex-direction: column;
  }

<div class="dashboard-body">
  <p>You don't have any sales data.</p>
  <p>Please add a new book or upload CSVs.</p>

  <div class="btn-group">
    <a href="#">
      <input class="add-new-book-btn" type="submit" value="Add New Book">
    </a>
    <a href="#">
      <input class="add-new-book-btn" type="submit" value="Upload CSV">
    </a>
  </div>