I have the same problem, that is to say that I want only the first name that appears on the Form.
The plugin that you were talking abouta year ago is not a free one. I installed a highly rated free plugin called Smart Custom Display Name. Where you should put “Display name publicly as” I put Keresztnév (I am working on a Hungarian site so Keresztnév means First name) but nothing has changed, ont the form here https://www.leanyvirag.hu/hirek/ you can see my full name Chopitel Emese . What is the solution please ?
Here we see you're posing as a guest but the option is related to the registered users. So we don't follow you well. Please record a video where we can see the issue.