Contact Custom field in chat_launch.php

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I have created a custom field, contacts.c$contact_source and want to add it in the chat_launch page.

I've updated the code as below:

<rn:widget path="input/TextInput" name="contacts.c$contact_source" required="true" />

But, it gives me the below error:

Widget Error: standard/input/TextInput - c$contact_source is a read-only field and cannot be used with an input widget.

 

What should I do inorder to achive the required,

 

Thanks,

Bipin