How to prepend a text of string to a content_type's field? For my assignment I have to write a module that when enabled will prepend a text of string "Hello World" to the body of a text field of hello_world_article content type.

I first tried to use Drupal 8 config export to try to set a default value (hello World) by using a yml file. However I get an error enabling the module that " field.field.node.hello_world_article.body already exists in active configuration." 

My yml file is:

uuid: 42c51471-d897-4cb1-ac37-b2b6cddc6f0e58
langcode: en
status: true
dependencies:
  config:
    - field.storage.node.body
    - node.type.hello_world_article
  module:
    - text
_core:
  default_config_hash: e_0gHBXrh9sHHe-XksjIYpOmFEiugHbtoJdYPxbftAI
id: node.hello_world_article.body
field_name: body
entity_type: node
bundle: hello_world_article
label: Body
description: ''
required: false
translatable: true
default_value:
  -
    summary: ''
    value: "&lt;p&gt;&lt;em&gt;Hello World!&lt;/em&gt;&lt;/p&gt;\r\n\r\n&lt;p&gt;&amp;nbsp;&lt;/p&gt;\r\n"
    format: basic_html
default_value_callback: ''
settings:
  display_summary: true
field_type: text_with_summary


How else would I get about approaching this problem. Help would be much appreciated.
Create a template override for the field. Because it sounds like a class assignment, I'll leave the actual coding to you.