I'm trying to add required fields to hidden input. But I cannot get it to work.

My xpath:

attribute name="t-att-value" separator="," add="first_name,last_name"

I get:

type="hidden" name="field_required" value="('phone,name', None, None)"

But I need:

type="hidden" name="field_required" value="phone,name,first_name,last_name"

Can this be done without using replace? And how?

Thanks!