VSTS deployment variable
While you can use the Release Description field the moment you Start a Release



which fills in the $(Release.ReleaseDescription) variable, I discourage this approach.

The whole point of a Release Pipeline is to automate and having users inputting data is against this approach.
If you do not trust VSTS for Secrets, you can store them in Azure KeyVault or similar and pull them down while releasing. It is not a chance that there are Tasks to facilitate this.