Blog search function

Hi there
On my site I have a blog and I want to add a search function, that only searches within this blog. I have tried different approaches but I can’t seem to find the right solution for this since the search always ends up finder all kinds of other stuff as well.
If you look at this blog here: https://www.weensu.dk/2019/05/19/baeredygtighed-et-begreb-med-tyngde/ you will see that at the left (bottom on mobile) I have included most read blog post in the widget area. I would really much like to add a search within blogs as well.

Does any of you bright people have an idea for some code or maybe a good plugin – paid/free.

Thanks

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