Multiline string in Azure Pipelines
Shayki Abramczyk pointed out the key to the this error.
Just put one ' in the start and the end of the msbuildArgs
  without having to configure for each MSBuild argument
As test, following YAML work for me:
- task: VSBuild@1
  displayName: 'Build solution YourSolution'
  inputs:
    solution: $(solution)
    msbuildArgs: '/p:DeployOnBuild=true /p:WebPublishMethod=Package /p:PackageAsSingleFile=true /p:SkipInvalidConfigurations=true /p:DesktopBuildPackageLocation="$(build.artifactStagingDirectory)\WebApp.zip" /p:DeployIisAppPath="Default Web Site"'
    platform: '$(buildPlatform)'
    configuration: '$(buildConfiguration)'
Note: The variable $(solution) should point to a specific solution .sln or project .csproj file instead of **\*.sln. If you have one more solution in your repo, you may get the error Only one project can be specified.
Update:
but I don’t want super long run-on line as in your answer provided. I
  want to split across multiple lines!
If you do not want to super long run-on line as in MSBuild arguments, you could split them directly, but pay attention to indentation, like:
- task: VSBuild@1
  displayName: 'Build solution YourSolution'
  inputs:
    solution: $(solution)
    msbuildArgs: '/p:DeployOnBuild=true
                  /p:WebPublishMethod=Package 
                  /p:PackageAsSingleFile=true 
                  /p:SkipInvalidConfigurations=true 
                  /p:DesktopBuildPackageLocation="$(build.artifactStagingDirectory)\WebApp.zip" 
                  /p:DeployIisAppPath="Default Web Site"'
    platform: '$(buildPlatform)'
    configuration: '$(buildConfiguration)'
As test, it works fine.
Hope this helps.