Your command will not work because since sqlcmd is not a Cmdlet. This means sqlcmd will only return text, instead of a PowerShell object.

To solve this, you should instead use a Cmdlet from a module like the Microsoft official sqlserver or the community made dbatools.

sqlserver:

# Run this once to get the module if you don't already have it (requires PowerShell Get)
Install-Module sqlserver

# Invoke this every time you begin a new instance (new terminal window or new script)
Import-Module sqlserver

Invoke-SqlCmd -ServerInstance 'yourserver\instance' -Query 'SELECT * FROM whatever' |
    Select-Object -Property ColumnNameHere

dbatools:

^{Disclaimer: I have made non-code contributions to dbatools in the past but I am not officially affiliated with them}

# Run this once to get the module if you don't already have it (requires PowerShell Get)
Install-Module dbatools

# Invoke this every time you begin a new instance (new terminal window or new script)
Import-Module dbatools

Invoke-DbaQuery -SqlInstance 'yourserver\instance' -Query 'SELECT * FROM whatever' |
    Select-Object -Property ColumnNameHere