What is the best way to pass some not predifined string to model construct in Magento 2 with di.xml? I have an api class which requires incoming params like service url, login and license key which i want to take from config section.
The correct answer would be, you dont do this. You have at least a data Object.
But Maybe you are more searching for the Configuration parts already existent in Magento1. You can use them like here and like in Magento1 you can create a default entry for this configs, so you dont need to add a real input field in the system config.