There are a couple of ways you can add links to other peoples’ websites. Since you don’t control their website but they do, then you can’t really go in and add links unless they want it or allow it. So you will need to follow some digital-PR steps to “convince” them to add a link to your website.
Another place online you can add links to your website is blogs and forums that talk about the same stuff your website is about. Our website is about law services so we should find blogs and forums that talk about law. After we do and we pinpoint the ones that are of high quality and seem important, then we need to join the conversation. Be careful here, you must join the conversation, not spam it with injected links or you will be kicked out in no time.
When you find a blog that talks about i.e. a trial that went wrong, then you can go below it in the comments, see what people are talking about and add your own knowledge there, comment and converse and in one of your comments say “read about criminal law and how we see it” adding your link to your website. Be creative and polite when doing this or the community of the blog/forum can be really irritated. Never go in and just paste your link as this is considered one of the worst types of spamming online, you will be permanently blocked from that blog/forum.