Add input dynamically with REST API

Hello, i'm using the adobe sign rest API to create an agreement and i have been experimenting in multiple ways how to add a signature field in a specific page without success, also want to add other text inputs through the API.

I have been trying to use a combination of the create agreement and the PUT formFields endpoint. However every time i change the agreement state to IN_PROCESS it overrides my formFields with some auto generated ones.

can it be done? or i'm wasting my time trying?

Thanks in advance.