Add the task to the schedule, and set it as fixed work. Assign the resource and specify the number of work hours. Then do the paste link to the Project start and Project Finish. However - are you sure it's fixed work? If the duration of the project increases, shouldn't the work increase as well?

Thanks Julie,

I was doing things the wrong way around!

The plan is for a project estimated to start much later this year so at present I'm anticipating a start date change (which will then drive a change to the end date) but with the duration remaining the same.