bike_index_recoveries | An app to show a list of bikes recovered by BikeIndex.org

Is kind of a pain in the ass. Everything you need to know comes from here: https://developers.facebook.com/docs/facebook-login/access-tokens/. Namely that you need a page-token to update a page, and that in order to get a page token, you have to get a user-token and then ask for me/accounts. I'll try to lay out the process I used here.
Sign up the application at https://developers.facebook.com/
Open a new tab pointed to the Graph Explorer tool https://developers.facebook.com/tools/explorer
Use the Application drop down to select your application
Click Get Access Token and don't click any boxes, just do Get Access Token
If you want to, do a GET to /v2.0/me (optional)
Click Get Access Token again. This time, select Extended Permissions and then the boxes publish_actions and manage_pages, and then Get Access Token.
Now you have a short term user-token for yourself in the Access Token field in the Graph Explorer. If you do a GET to /v2.0/me/accounts you'll see what pages you have access to, access tokens for you to use those pages through the API, and your permissions on those pages. However this is a short term (a few hours) access token to the page. We want a forever page-token.
To get a forever page-token, you have to do the GET to /v2.0/me/accounts using a long-term user-token. To get that, whip out curl or your preferred method of receiving JSON, perform the following at https://graph.facebook.com GET /oauth/access_token? grant_type=fb_exchange_token& client_id={app-id}& client_secret={app-secret}& fb_exchange_token={short-lived-token} where {app-id} and {app-secret} are on the page you still have open from step 1, and {short-lived-token} is what you got in step 6.
The access_token you receive in the JSON response is your long term user-token (as you can see from its 60 day expiry time, listed in seconds). Copy this access_token into the Access Token field in the Graph Explorer
Use the explorer to GET /v2.0/me/accounts again. This time the access token is a page-token that will never expire. Save this to your secrets file that you'll use in the app to post to facebook. If you'd like to verify that it won't expire, you can use curl or whatever to GET graph.facebook.com/debug_token? input_token={token-to-inspect} &access_token={app-token-or-admin-token} where {token-to-inspect} is that new page-token and {admin-token} is a viable user-token for an admin of the app (you, presumably, because you just created the app) (use your access token from step 6 or 9 here). Don't worry about app-token. This should give you some JSON with an issued_at UNIX time and an "expires_at": 0