This is the Server Action that was created for the demo:

Once it is saved, it will have an ID (you can find it in the URL) and this is one way to identify it to attach it to a button.  If this Server Action is created as part of a module, then it would also receive an External ID which could be used to attach it to a button.

The ID method of attaching it to a button is like this:

(The ID in this database was 594, so change that based on the ID of the Server Action when saved in your database)

<xpath expr="header" position="inside">
  <button class="btn btn-secondary" name="594" type="action" string="Check Inventory" />
</xpath>