How to Set /Change HF Service Account

It appears that some of my method calls from HF fail because of the account HF is running under but I am not able to find how to set HF service account to “Network Service” as an example. Any ideas? My method works fine if called by API or under user account. It fails with “Object reference not set to an instance of an object” on the first line of the method. I am not even able to debug it.