
Explanation:
Step 1: In The Application Object Tree (AOT), right-click the CustTable form.
Step 2: Select Create extension.
Step 3: Rename the new CustTable extension
Note: To create an extension (in general terms), follow these steps:
1. In the Application Explorer window, expand the AOT node.
2. Expand the node for the element that you want to extend. For example, in Classic view, extend the Data Models node, and then extend the Tables node to view all available tables.
3. Right-click the element that you want to extend.
4. Select Create extension to add an extension to your current project. Select Create extension in new project if you want to add the extension to a new project. The element will be added to a project in the Solution Explorer window, and it will open in the element designer.
Reference:
https://docs.microsoft.com/en-us/dynamics365/business-central/dev-itpro/developer/devenv-extension-example