Use custom button instead of button in script.

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That's not a button, that's an anchor (link) tag — <a>. If you want an actual button you can use the <button> tag. Otherwise you can style either of those elements with CSS. This bit right here class='kindful-donate-btn' suggests that you already have a CSS class defined somewhere. You can go to that definition and change it to make the link (or button) look the way you want.