I believe you will need to use an SPFx(SharePoint Framework extension) to modify Start form in SharePoint Modern UI

There is no way to customize de start form of a Workflow (with a nintex forms, this feature didn't exist).

I imagine you want to customize the start form of a site workflow.

An ugly solution will be : 

Use a list with a custom forms (and field that you need to pass to you workflow) that start a new list workflow.

In this workflow :

        - kick your site workflow with your parameter values

        - Delete you item

And if you realy realy realy want to customise the start form of you workflow, the only way is to develop something with custom code that will manipulate the start page.

No, workflow start form customization is not available.

Thank you all for your reply. Which is a pity.

@nintex: any chance this will be added in the near future?