How can create a block that get a field value and render it?
If you use setConfigurationValue(), put the value as parameter in the function:

$this-&gt;setConfigurationValue('default_LinkA_url', $form_state-&gt;getValue('block_LinkA_url'));


Or better set the value in the configuration array like this:

$this-&gt;configuration['default_LinkA_url'] = $form_state-&gt;getValue('block_LinkA_url');


To display the url use the render element link:

https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Render!Element!Link.php/class/Link/8.2.x