Launch custom code via tasks in Inno Setup

You don't need to define your own wizard page. You can just add them to the additional tasks page.

[Tasks]
Name: associate; Description:"&Associate .ext files with this version of my program"; \
    GroupDescription: "File association:"

[Code]
function NextButtonClick(CurPageID: Integer): Boolean;
begin
  Result := True;
  if CurPageID = wpSelectTasks then
  begin
    if WizardIsTaskSelected('taskname') then
      MsgBox('First task has been checked.', mbInformation, MB_OK);
    else
      MsgBox('First task has NOT been checked.', mbInformation, MB_OK);
  end;
end;

Credit goes to TLama for this post.

You do that by adding a custom wizard page that has check boxes, and execute the code for all selected check boxes when the user clicks "Next" on that page:

[Code]
var
  ActionPage: TInputOptionWizardPage;
  
procedure InitializeWizard;
begin
  ActionPage := CreateInputOptionPage(wpReady,
    'Optional Actions Test', 'Which actions should be performed?',
    'Please select all optional actions you want to be performed, then click Next.',
    False, False);
    
  ActionPage.Add('Action 1');
  ActionPage.Add('Action 2');
  ActionPage.Add('Action 3');
  
  ActionPage.Values[0] := True;
  ActionPage.Values[1] := False;
  ActionPage.Values[2] := False;
end;

function NextButtonClick(CurPageID: Integer): Boolean;
begin
  Result := True;
  if CurPageID = ActionPage.ID then begin
    if ActionPage.Values[0] then
      MsgBox('Action 1', mbInformation, MB_OK);
    if ActionPage.Values[1] then
      MsgBox('Action 2', mbInformation, MB_OK);
    if ActionPage.Values[2] then
      MsgBox('Action 3', mbInformation, MB_OK);
  end;
end;

The check boxes can either be standard controls or items in a list box, see the Inno Setup documentation on Pascal Scripting for details.

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If you want your code to be executed depending on whether a certain component or task has been selected, then use the WizardIsComponentSelected() and WizardIsTaskSelected() functions instead.