I was dragging my feet on setting up metaminding for my pre-existing meta goal because I thought it would be fiddly … but it’s not! So, to reiterate from the instructions at https://forum.beeminder.com/t/beeminder-push-to-beeminder-aka-rudimentary-meta-integration-aka-disintermediate-ifttt-for-foo-days-goals/
Just go to the Settings tab for your main goal, scroll down to the DATA section, look for the Webhook field, and paste in the URL of your meta goal. That’s it. Now your meta goal will get an automatic +1 when you add data to your main goal.