How do I add a template suggestion for a Views block?

Inside sites/default/services.yml add config like this:

parameters:
  twig.config:
    debug: true

(and clear the cache) and it will turn on twig debug mode, which will give you information (inside html comments in page source) on what templates are used and possible template name suggestions to override those templates.

You need to name your template based on the rules for Views templates.

For example, "views-view--partners--partners--sales.html.twig" would mean:

• viewid: partners
• view-display-id: partners
• fieldid: sales

If this is correct, then all you need to do is clear the cache and reload; the template should be picked up automatically.