I don't think you can do that. Capitalism!!!! Mojang will make some money from people buying java edition. You are gonna have to pay like it or not :/

Mojang / Microsoft seems to be trying to steer people towards getting Minecraft Win 10 / bedrock edition and not so much to Java anymore. You can/could get Win 10 for free if you have Java, but not the other way around. :( I'm afraid you'll have to purchase the Java version separately.