Unlike other search engines, DuckDuckGo doesn’t offer any kind of webmaster tools.
Duckduckgo also not offers URL submission.
So, Is there any way to submit your site to DuckDuckGo. Yes, they don’t have any kind of a crawler that is going to crawl your website. To index your site, you have to rely on the search results from other search engines and come up with a way to index your site.
There is no need to submit your site to duckduckgo also there is no way to submit.
As duckduckgo automatically index your site.
So, It might take some time to show your site on duckduckgo search Engine.
I have made a Youtube Tutorial on Google, Bing and Yandex Webmaster Submission. Have a look at that.