First let me explain Viber Chat Extension. Chat extension is a Viber feature which allows you to search and share content during the chat just by clicking on the gif button.
Well the options in chat extension are very clear and easy but still if you don't find what you are looking for in chat extension. Go to the settings and try to find what you are looking for.
But if you still can't find what you are looking for. Write an email to Viber and tell your issues and ask for guidance and help and they will resolve your issues.