Just replace :projectSlug with the slug of your Project (e.g. webpack for https://donatepr.com/webpack). The verb can either be "donate" or "contribute".
If you want to add a donate button to a blog post, you can load an image version of the logo and then link to the donate page of your project.
On Medium, download the image then upload it to your post. To link it, select the image and press CMD+K (or CTRL+K on Windows). A pop up menu will show up where you can enter the URL to link the image to (see Medium support page). Here is an example.