You can embed a forum into your Yola website by getting your forum embed code and then following these instructions to insert the embed code into your Yola website:

Step 1: Get your embed code

1. Log in to your Website Toolbox account.
2. Click the Integrate link in the main menu.
3. Click the Embed link.
4. Click the Embed your forum link.
5. If a Create A Subdomain First button appears, click it to connect your forum to your website domain so that it remains embedded when accessed from the Safari browser, or click Get Embed Code to come back to this step later.
6. Click Get Embed Code and copy the embed code to your clipboard.

Step 2: Add the embed code to your website

Once you have the embed code, you can add it to your website by following the steps below.

1. Visit Yola.com
2. Log in to your account.
3. Go to your website where you want to embed the forum.
4. Click Edit Site.
5. Click Pages.
6. Click the + icon.
7. For Page Name, enter Forum (or whatever you would like to name the page which will show your forum).
8. Check the check box next to Display in navigation.
9. Click Submit.
10. Click the + icon to add a block.
11. Search HTML in the search box.
12. Scroll the page down and click the HTML icon.
13. Hover the mouse and click the icon.
14. Paste your forum embed code.
15. Click Submit.
16. Click the Save icon from the top.
17. Click Publish.
18. View your Yola website. A Forum link will now appear in the navigation of your website. Clicking that will open a page with your forum embedded in it.

Here is a reference tutorial from Yola with instructions related to adding HTML on your Yola website.