When an agent manages chats and conversation within Zendesk, they have two main ways to offer a Snapcall to the customer:
Paste a Snapcall URL within the conversation: this is the simplest way to start a call as the customer will see the link on the chat. All they need to do in this case is to click on the link, so that they'll be redirected to the call itself.
Send a call card: agent can also send a card within a Zendesk conversation. This is the best way to initiate a call as it gives more context to the customer about what has happened. The card specifically states that if the customer will click on the button within the card itself, they will be redirected to the call and so they have the possibility to speak directly with the agent.