For those who decided to code first there are some approaches to generate the Swagger/OpenAPI definition for an existing API:
1. Call methods of your API from Swagger Inspector and auto-generate definition based on the history of calls: https://swagger.io/docs/swagger-inspector/how-to-create-an-openapi-definition-using-swagger/
2. Generate definition based on metadata added to your API project with a special tool. Please refer to this document for details: https://swagger.io/blog/api-development/swagger-api-documentation/
