Ask them how do they automatically ""post" the payloads to end point without using any tool
"Post" automatically isn't humanely possible!
You just need to understand the architecture of framework, REST or SOAP. Also there are multiple frameworks are there in market implementing any of these.
For example, In your new company they might use Jersey RESTfull services, You just need to know how to use jersey client to invoke API, further you have to parse response and validate. So in short get the name of framework, refer its standard documentation.
Hope it helps!!