Customizing create topic form to add more custom fields, what does add_to_serializer or register_custom_field_type do?

[net_deamon]

I can change the height of create post box to include more custom fields.

But I am not able to figure out how to pass the value from custom field to TopicCustomField.
I tried browsing many plugins of discourse in github. Discourse-tagging ( depreciated) is the closest one that matches. I have posted snippet of that code in the question, … How do we pass the value from handlebar template to discourse topic?