Newsletter subscribtion HTML CSS JS

I am working with Blogger and I am a beginner. I want to add this newsletter subscription to my website: https://ift.tt/2p7YqzF
But I can only add the HTML and CSS code, not JS. And after I have done that, when I enter an email address and press enter, it refreshes the page. Do I have to change something in the HTML/CSS/JS text? Thank you very much in advance.

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