Idea does not provide the ability to create tasks manually from the UI.
But if the 'Allow Effort Task Creation' checkbox is enabled at the administration side, adding a team member to the Idea Team page creates an effort task (~rmw) and the team members can enter timesheets against this task.
However, in certain cases when idea is created using REST API, adding a team member is still not creating the effort task and hence the resource is not able to enter timesheet against this Idea.
The root cause of this issue was a custom process which was run against the idea creation causing it to not allow the effort task creation.