There are 2 options:
Create an integration user under an email address owned by the client. The integration user should be a member of the workspace. Then, a permanent access token should be requested for the integration user. From there, your scripts can use that access token to make requests for data in Zengine. If what you're trying to do is sync data from Zengine to another service, you can create a web hook that calls your script when records are updated in Zengine, then have your script fetch the records and push them to the other service.
Alternatively, you could create a Zengine plugin with a backend service. Then you could setup a web hook that calls that backend service instead of a script on your server. The backend service would get an access token automatically. The backend service would then make requests to the Zengine API and send the data to the external service.
Both of these approaches would be a similar workflow: 1. Record is updated in Zengine. 2. Web hook is fired. 3. Your code fetches data from Zengine API and pushes it to external service.
The difference is that approach one would run on your server and require a permanent access token, while approach two will run on our servers and will automatically get an access token. Approach one gives you full control over your code and the language you use. Approach two will require learning how our plugin system works and writing your code in javascript and node js using our frameworks.