href link assigning callback action - ajax
For anyone needing help on this, as the information out there seems to be either related to forms only or seems rather over convoluted... ajax callback can be added to normal links, by defining the link as outlined above (in the question) - but adding the '/nojs/' prefix to the Url defining portion of the code and adding the class 'use-ajax' in the attributes section of the code e.g. :

// Define the link.
$url = Url::fromUserInput('/nojs/ajax_call_url');
$the_link = Link::fromTextAndUrl('Submit Application',$url);
$the_link = $the_link-&gt;toRenderable();

$the_link['#attributes'] = [
  'class'=&gt;'use-ajax',
];

// Add the link to your render array. 
$output['link1'] = array(
    "the_link"=&gt;$the_link,
);


Now in the 'name of your module'.routing.yml file add the route to where your ajax callback function is... as '/nojs/' is used (as it has to be by the looks of things) it means your path must be defined with the prefix '/ajax/' followed by the path of your choice e.g.:

path: '/ajax/call_ajax_url'


the _controller attribute in the routing.yml file, will be defined to point to where your ajax callback command is defined... e.g. will point to whatever object and function holds the 

    $message="hello";
    $response=new AjaxResponse();
    $response-&gt;addCommand( new HtmlCommand("#current-msg",$message);
    return $response;


Hope this helps someone - as it took a while for me to find the info necessary to resolve this issue.