Hi Nur,
You need to amend the behaviour of the Additional Assignee list field in order to add Location of the user as a criteria when you need to make the selection. First you need to make sure that Location field from User table is populated and the appropriate user are mapped against the desired locations. You might need to configure Location to include Singapore and Indonesia.
Once this is done, you need to change the reference qualifier of Additional Assignee list within the Dictionary Entry Override as project form is extension of task, if you amend the dictionary entry it will affect the behaviour of this field across the platform, not just on the project form
Unfortunately I don't know what script goes to also include location in the ref qualifier but hope this will help you as a starting point and perhaps there are other posts in the Community who might give you a clue on how you need to build that line of code
