how to fix start date, end date, work, and let the units float?

[pocketgui]

Hello. I have the following question:
How can I fix the start date, end date, and work for a task, and let the units adjust to meet the criteria?

I want to link these tasks to project start and project finish.

Alternatively, how can I fix the start date, end date, units, and let the work adjust to meet the criteria?

Thank you