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
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Edit Environment Variables
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Create Variable named DOTNET_CLI_TELEMETRY_OPTOUT
with a Variable value of 1
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Save by Hitting OK and OK again.