You must understand that @Section is a blade tag. Blade tags are processed inside PHP. Also, the blade syntax does not accept the parameter onload as you want.
Also, either on HTML/JS you can't use onload as you want.
You have two alternatives:
1 - Put a script tag after the section:
@section('content')
  <!--PAGE CONTENT...-->
  <div>
   <h1>Something...</h1>
   <select name="sel"></select>
  </div>
  <script>
    // Note that this function must have been declared before
    // this script tag, otherwise it will log an error:
    // "Uncaught ReferenceError: MyFunction is not defined"
    MyFunction(); 
  </script> 
@endsection
In this solution, the function will be called right after the browser loads this piece of html, but other parts of the html may not be loaded yet.
2 - Put a script tag watching for the load event on document:
<script>
    $(document).load(function() {
        MyFunction(); 
    });
</script>
The advantage of this method is that it will only be called after the entire page being loaded, so the order of the <script> tags doesn't matters (except that the jquery call must be after the jquery script tag.