Access right problems while using RestAPI

We are working with RestAPI to make our own request system and 1IM (ver 7.1.2) works together.

3 tables - Person, ITShopOrg and PersonWantsOrg - will be will be read by request system, and write access is required for data insert on table 'PersonWantsOrg'.

There seems to be 2 ways to solve:

1. Create a system account with access rights to those 3 tables.

2. Create an AD account to do the same thing.

But we are not managed to get it work, because:

For option 1, system accounts do not have write access to PersonWantsOrg, an Employee account is required.

For option 2, Employee / AD accounts do not seem to be able to grant access to read or write tables directly via RestAPI.

Could anyone advise if we did anything wrong, or there are any other ways to achieve?