smooth-scroll-to-anchor.js

jQuery(document).on('ready', function(event) {
	
	// Scroll smoothly to anchor
	jQuery('a[href^="#"]').on('click', function(event) {
	    var target = jQuery(this.getAttribute('href'));
	    if( target.length ) {
	        event.preventDefault();
	        jQuery('html, body').stop().animate({
	            scrollTop: target.offset().top
	        }, 1000);
	    }
	});

});

Here is a doc which explains how custom JS can be added to your site - https://wpastra.com/docs/add-custom-javascript-code-astra/