How do I create a custom button in the git Message field of the VS Code Source Control tab?

I am creating a VS Code extension. I want to add a custom button to the Git message field of the VS Code Source Control tab?

To be specific location, please see the screenshot below:

I want to add a custom button to this location. Just like GitHub Copilot did.

So my question are:

1. How to add button to it?
2. How can I put generated message to the "Message" field?

You need to use the contribSourceControlInputBoxMenu API proposal. See also its issue ticket: SourceControl - SourceControlInputBoxValueProvider API proposal #195474.

The API proposal is current empty. At a previous point in time, it looked like this:

/*---------------------------------------------------------------------------------------------
 *  Copyright (c) Microsoft Corporation. All rights reserved.
 *  Licensed under the MIT License. See License.txt in the project root for license information.
 *--------------------------------------------------------------------------------------------*/

declare module 'vscode' {
  // https://github.com/microsoft/vscode/issues/195474

  export namespace scm {
      export function registerSourceControlInputBoxValueProvider(sourceControlId: string, provider: SourceControlInputBoxValueProvider): Disposable;
  }

  export interface SourceControlInputBoxValueProviderContext {
      readonly resourceGroupId: string;
      readonly resources: readonly Uri[];
  }

  export interface SourceControlInputBoxValueProvider {
      readonly label: string;
      readonly icon?: Uri | { light: Uri; dark: Uri } | ThemeIcon;

      provideValue(rootUri: Uri, context: SourceControlInputBoxValueProviderContext[], token: CancellationToken): ProviderResult<string | undefined>;
  }

}

You could take a look at the GitHub Copilot Chat extension's minified code as an example (download VSIX, rename filename extension to ".zip", unzip). In its extension manifest, it has this:

"scm/inputBox": [
  {
    "command": "github.copilot.git.generateCommitMessage",
    "when": "scmProvider == git"
  }
]

See also https://code.visualstudio.com/api/advanced-topics/using-proposed-api.

When the API still existed, the way to change the value of the input box was vscode.extensions.getExtension('vscode.git')?.exports.getAPI(1).repositories[0].inputBox.value = newvalue;.

Note that later versions of GitHub Copilot started using the scminput language instead of the scm/inputbox menu contribution point (see the github.copilot.enable setting).