Section 21 Notice - Does Tenant Sign?

Possession - Section 8 and Section 21

On previous courses I have attended the RLA have advised serving a Section 21 Notice at the start of the tenancy. The theory behind this being that the tenant will sign it then but would be less likely to sign it if served at a later date if they did not want to leave.

One of my properties is rented out through local agents. I am now selling the property and have given the tenants notice. The Letting Agents have served a Section 21 Notice on my tenants. However, there is no provision on their notice for the tenant to sign anywhere.

Does the tenant have to sign a Section 21 Notice or not?

Thanks