You will first have to create a Page using SDK( I am using www.google.com):

<PageDefinitionSpec
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  ID="31128227-2a2b-47fd-b740-7af31146c182"
  Name="External Website"
  Description="External Website within CRM"
  Author="Author"
  ExpressionDataFormID="00000000-0000-0000-0000-000000000000"
  NameUIOverride="External Website"
  xmlns="bb_appfx_pagedefinition"
>
  <PageHeader Caption="Website within CRM" />
  <SummarySection ID="fbc31098-cb5d-43f2-a7db-d30e1630b416">
    <WebPage Url="www.google.com">
      <WinUIOptions />
    </WebPage>
  </SummarySection>
</PageDefinitionSpec>

Then the task will look like this:
<TaskSpec
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  ID="5a54cf57-0165-447b-9831-1e91c5f9f937"
  Name="External Website"
  Description="External Website"
  Author="Author"
  FunctionalAreaID="00050ce8-8abb-4a86-b9af-70a035ce0582"
  Sequence="0"
  xmlns="bb_appfx_task"
>
  <ShowPage PageID="31128227-2a2b-47fd-b740-7af31146c182" TabID="00000000-0000-0000-0000-000000000000" xmlns="bb_appfx_commontypes" />
</TaskSpec>

For this test the Functional Area Id is the Constituents.  You can change this to what you need to have the task show up where you need it.