How to setup anchor links in the menu
One-Page Site Option
You have the option to setup your site as a traditional multi page site, or as a one-page site. In the one-page style, you can still include links to other pages as well, such as the blog.
If you'd like your site to be One Page style, when building your site just add all of your widgets and content onto one page. For each section you can use the Elementor widget ID option, or the Anchor link widget to add your anchors. You can use any text you like, without spaces. For example: aboutus
Once you have that in place, you can create your navigation Menu under Appearance > Menus
On the left side, click Links, and input the URL to your page, including you anchor with a hashtag before it, ie: http://www.yourdomain.com/#aboutus
Once all your links are added, go to the top and click Screen Options. Ensure the box is checked for CSS Classes.
In each of your one-page links, click the down arrow next to Custom and insert the class:
Elementor has a video showing how to use the Anchor Link widget option: