API to fund a task a sub-task level

Looking for some assistance with locating the correct API to fund a task a sub-task level

Project

        TT1 (Total Amount A+B+C)

LL1 (Lower level subtask)    <-->  Resource e.g. Salaries  <--> Budget cost A

LL2 (Lower level subtask)    <-->  Resource e.g. Equipment  <--> Budget cost B

LL3 (Lower level subtask)    <-->  Resource e.g. etc   <--> Budget cost C

The bit we need to create is the Resource and Budget Costs for the Subtasks.

The project, top task and sub task (as well as the agreement details) will already have been created by the process earlier on, but we are struggling with finding the correct API to fund the sub-task.Any ideas?