I want to use the Buffett code WebAPI from curl.

Because you authenticate with the API Key, set the API Key in the x-api-key header as follows:

$curl-s https://api.buffett-code.com/api/v2/indicator\?tickers\=2120\&from\=2017Q1\&to\=2018Q1\
    --header'x-api-key:<Your API Key>'  

If the API Key is correct, a response in JSON format is returned.