Pulling of selected field

hello and good morning

I created a custom field called “section” in task DOCTYPE using a field type “select”. Going below the form is a section called material with a table having a column called purchase order.
My request

when i create a task and select a project and then go to section eg “B. EXCAVATION” on section and go down the form and select the link to generate a new purchase order i want “B. EXACAVATION” and project name be pulled from the task to the appropriate field

on the purchase order.

how do i go about this