So when I create a detailed issue through the "+" button on the left I can't add it to an Epic. Only after I created it. I think it would be a nice little feature ;)
Welcome to Atlassian Community!
On JIRA Next-Gen, you are correct to say that the Epic-Link field is not visible in any of the issue screens (Create/Edit/View).
In fact, we agree with you that this feature would be great to provide the customers with the possibility to link Stories to Epics directly from the Story side, so we created the following suggestion:
Feel free to vote and watch the suggestion to increase its priority and also receive notifications about any updates.
Let me know if you still have any questions.
We definitely would like to have this feature - we would like to be able to trigger notifications by epic to specific slack channel, but because they also require some sort of action trigger (like create issue), they won't work because the epic can only be added after the issue is created and the actions that trigger the integration don't include being added to parent epic or ticket is edited.