post https://api.factorialhr.com/api/v1/custom_fields/values
As described, a Custom Field Value can be thought of as an answer/response to a Custom Field these answers/responses belong to the entity that inputed the values e.g. an employee who answered the t-shirt size question by filling it out in their factorial dashboard.
This endpoint allows you to create values for custom fields. It requires an instance_id which refers to the id of the entity that owns this Custom Value e.g. an Employee for which the employee_id will correspond to the instance_id. It also requires a field_id to reference the Custom Field which this value is related to.
You can think of Custom Fields and Custom Values as questions and answers.