If you go to https://duckduckgo.com, you'll be able to find out if your site has been indexed and available in their search results.
In the search box, type site: followed by your domain name then press Return or click the search icon.
If your site is indexed, you'll see a list showing the pages that DuckDuckGo knows about on your web-site.
The chances are, if your website has been around for a while, you'll see plenty of results already. If not, you'll need to submit your site to the indexing service.
If you look around the DuckDuckGo website, you'll notice a complete lack of links to submit your site. This is because they use the data services of other companies to get their web site results, specifically Bing, Yahoo and Yandex. They also crawl over 400 other directories and databases to gather their results.
If you want to fast track your appearance in DuckDuckGo's results, it helps if you submit your web-site to Yandex.
Please note: If you've already submitted your site to Bing and Yahoo, (or you've been indexed by them), you probably won't get any benefit from submitting your site to Yandex too (although it can't hurt to appear in Yandex's databases too).
Next: How to add your site to Yandex using their webmaster console
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