First option is to open a Power Shell or Command Prompt and type, think it may need to be an admin prompt.

set DOTNET_CLI_TELEMETRY_OPTOUT=1

As a secondary option you should also be able to do this from the GUI by doing the following.

Search for Environment Variables

Edit Environment Variables

Create Variable named DOTNET_CLI_TELEMETRY_OPTOUT

with a Variable value of 1

Save by Hitting OK and OK again.