i have an existing custom workflow that is being used for Approvals. i’m wondering, is there any way to click the Approve button through an API call or other automated process?

NetSuite has a workflow SuiteScript module that we can be used to click the button with a script. Potentially, you can deploy a SuiteLet or RESTLet with code to click the button. The SuiteLet or RESTLet can be initiated with an API call.