Follow below instructions:
1) Open Terminal
2) Nevigate to magento project root directory
3) Create Auth Tocken :
Run following commands and check url and username-passwords as per requirement.
curl -X POST "https://www.demomagento.com/rest/V1/integration/admin/token" \
-H "Content-Type:application/json" \
-d '{"username":"admin", "password":"Admin@123"}'
4) Export Schema in json format for postman
curl -X POST https://www.demomagento.com/rest/default/schema \
-H "Authorization: Bearer z4t8bqs7eaz90ch89g70et7kfmwzvzow" > demomagento.api.schema.json
Change Bearer token with one which was generated in 3rd Step.
5) It will export demomagento.api.schema.json file in the project directory use it in postman.