FlexForm with Plugin

Ref. link: ref: https://docs.typo3.org/m/typo3/reference-coreapi/main/en-us/ApiOverview/FlexForms/Index.html, 
https://docs.typo3.org/m/typo3/reference-coreapi/main/en-us/ApiOverview/FlexForms/T3datastructure/Index.html#t3ds

1. FlexForms can be used to store the data within an XML structure inside a single DB column.
2. FlexForms can be used to configure content elements (CE) or plugins, but they are optional so you can create plugins or content elements without using FlexForms.
3. Most of the configuration below is the same, whether you are adding configuration for a plugin or content element. The main difference is how addPiFlexFormValue() is used.
4. How the flex form works
   1. In the extension, a configuration schema is defined (you will see this in “Create flex form“) and attached to one or more content elements or plugins.
   2. When the CE or plugin is added to a page, it can be configured as defined by the configuration schema.
   3. The configuration for this content element is automatically saved to tt_content.pi_flexform.
   4. The extension can read the current configuration and act according to the configuration.

Let’s create the Flexform with xml extension as per the definition