Update on 2/6/2020 from @rrelyea:
Fixed with: NuGet/NuGet.Client#3206
Spec link: https://github.com/NuGet/Home/wiki/Add-nuget-sources-command-to-the-dotnet-CLI

Original text:

Moving from https://github.com/dotnet/cli/issues/5053#issuecomment-267794961 on behalf of @meichtf, @emgarten.

Steps to reproduce

try to add a username/ClearTextPassword in the dotnet restore --source command

Expected behavior

My nuget repository requires authentication and it is not possible to add a username/ClearTextPassword in the dotnet restore command for a source

Actual behavior

It's not possible to add a username/ClearTextPassword for a ..source

Environment data

dotnet --info output:
.NET Command Line Tools (1.0.0-preview2-1-003177)

Product Information:
Version: 1.0.0-preview2-1-003177
Commit SHA-1 hash: a2df9c2576

Runtime Environment:
OS Name: Windows
OS Version: 10.0.14393
OS Platform: Windows
RID: win10-x64