theming – pass twig variable in drupal_entity() in drupal 8

The first assumption is that you already installed twig tweak module.

I believe the problem is in getting the form_name part, not in rendering.
first solution :

{% set form_name = node.field_c.0.value|trim|striptags %}
{{ drupal_entity('webform', form_name) }}

so if your field is a single value
I would suggest use twig field value module. then you can easily use

{% set form_name = content.field_c| field_value %}
{{ drupal_entity('webform', form_name) }}

another solution is :

{% set form_name = content.field_c| field_value %}
{{ { '#type': 'webform', '#webform': form_name } }}