To tag a Dog Park in your post on the Global Feed, begin by clicking the large + button at the bottom of your screen:

Then click on the "Tag Park" button and select which park you want to tag in your post. Please note that to tag a park in a post, you must be following the park. You can learn how to follow a park here: How Do I Follow A Dog Park. Then, when you click "Post," your Post will appear on BOTH the dog park page you tagged and the Global Feed.