How to display particle background in header?

To use the particle background in the header first, you need to activate the extension in the Divi plus setting.
Please take a look at the below documentation link

https://diviextended.com/documentation/divi-plus/particles-background-extension/

Note: Here you can use the particle background in the theme builder header not with the default divi header.

Now move to the Divi >> Theme Builder >> Edit the header >> edit the section/row/module in which you want to add particle background >> Content tab >> Background >> Enable particle background