Use css in Nintex Form on SharePoint 2019 On-Prem

I'm new to Nintex form and try to add the form in SP2019 On-Prem modern site template (communication site). I create a form and want to add the customized css. By searching in forum and online, it mentions that in the form settings, there is a place to put customized css. But I don't see it in my form settings. Do I miss anything here? Or I should look for different place to set it up? Thanks! 




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