Drupal 8:How to add tags for custom content types
Go to /admin/structure/types and press on manage fields of your custom content type

Press on the +Add field button, and use Re-use an exiting field (the drop down on the right) to select the existing Entity Referenc: field_tags



Then go to manage form display, for tags change the widget to autocomplete (Tags style)

PS: I suggest you install admin toolbar if you haven't already.