It was previously possible to include any location when scheduling your Instagram post through Buffer. However, on November 2, 2020, Facebook deprecated the Places Search API, which means this is no longer possible.
Now, through the API, it's only possible to include the location that is associated with a Facebook Page. It can be your Page, or any other Facebook Page that has a location. You can do this by typing the name of a Facebook Page in the location field. If the Page has a location, it will be returned in the search results and you can select it.
📝 Check out this guide from Facebook if you'd like steps on adding a location to your own Page. It's important that this checkbox is ticked. If it's not ticked, this will result in the location being hidden on Instagram.
It's also important to note that this works a little differently than it did previously. In the past, the location on posts scheduled through Buffer would be displayed beneath the Instagram account handle. Now, the name of the Facebook Page will be shown beneath the Instagram account handle, and clicking on that will take the user to the location, with both "Top" and "Recent" posts tagged with that same location.