8 – How to customize a link

I installed the module "Private message" in Drupal 8.

There is a link to send a message that I want to transform into a Bootstrap button.

How to customize a link in Drupal 8?

Here is the HTML code of the link:

The developer of "Private Message" gave me this code, but I do not know what to do with it.

I need to create a custom module or put it in my_theme.info.yml ?

HOOK_node_view_alter (& $ build) {function
$ build['private_message_link']['#title']    = t (& # 39; Click on me. Click on me now! & # 39;);