If you want a link on a particular website, it doesn’t hurt to ask.
All they can say is “no.” Maybe you have something to offer the webmaster that you don’t even know about. Maybe you catch them in a good mood.
Most likely, you won’t even receive a reply, but you never know until you ask. And, when they do reply you might start a dialogue about what types of things webmasters are looking for in your niche in order to merit a backlink.
It may be different, depending on the area you are promoting.