First, create a Site123 Contact Form app on Common Ninja.
On the apps catalog, find the Contact Form and enter the Editor.
When you're done creating the Contact Form app, click on the Save Changes button at the right top corner of the editor.
If you don't have an account on Common Ninja, you'll have to register first.
When you're ready to add the Contact Form app to your Site123 website, click the Add To Website button at the top bar of the Editor.
On the window that opens, click the Copy Icon to copy the app embed code. In the next steps, you will paste this code into your Site123 [site_type].
Now, let's head to your Site123 website's editor.
In the Site123 editor, add a new section wherever you wish the Contact Form to show.
In the left section panel, click on Text.
From the Text Section options, select a Content Page section.
In the new section you've added, click the Edit button at the right top corner of the section.
You can delete the text, or add the widget to an existing text.
Click the Custom Code button at the bottom of the text section.
In the window that opens, click the Change button.
Now, in the HTML window that shows, simply past the Contact Form embed code you've copied before.
Click the Update button at the bottom of the HTML window.
Click on the Save button at the left bottom corner of the Content Page section, to save your changes.