I didn't know off of the top of my head so I used Bing.

The following is from http://stackoverflow.com/questions/30899586/how-to-get-stored-procedure-output-parameter-into-variable-using-powershell:

$SqlConnection = New-Object System.Data.SqlClient.SqlConnection

$SqlConnection.ConnectionString = "Server=myserver;Database=mydb;Integrated Security=True"

$SqlCmd = New-Object System.Data.SqlClient.SqlCommand

$SqlCmd.CommandText = "testsp3"

$SqlCmd.Connection = $SqlConnection

$SqlCmd.CommandType = [System.Data.CommandType]'StoredProcedure'; <-- Missing

$outParameter = new-object System.Data.SqlClient.SqlParameter;

$outParameter.ParameterName = "@answer";

$outParameter.Direction = [System.Data.ParameterDirection]'Output';

$outParameter.DbType = [System.Data.DbType]'String';

$outParameter.Size = 2500;

$SqlCmd.Parameters.Add($outParameter) >> $null;

$SqlConnection.Open();

$result = $SqlCmd.ExecuteNonQuery();

$truth = $SqlCmd.Parameters["@answer"].Value;

$SqlConnection.Close();

$truth;