How to create a custom form in composer?

It’s a great feature but it doesn’t full fill my requirement. I can see this is creating template and that plugged in create topic form which is different. But, I want to display an input tag in composer and while saving or submitting the form. I have to create a topic with some data. Basically, It will render two composer one by one first with input tag and then it wil render create topic form composer with pre-filled data which I will pass from first composer.

Yeah, I am doing this in custom-theme but I am not able to render an input tag in composer. I am looking for an way to do it.