Post-ToSlack.ps1

function Post-ToSlack 
{
    Param(
        [Parameter(Mandatory = $true,Position = 0,HelpMessage = 'Chat message to send')]
        [ValidateNotNullorEmpty()]
        [String]$Message,
        [Parameter(Mandatory = $true,Position = 1,HelpMessage = 'Slack webhook URL')]
        [ValidateNotNullorEmpty()]
        [String]$uri,
        [Parameter(Mandatory = $false,Position = 2,HelpMessage = 'Slack channel')]
        [ValidateNotNullorEmpty()]
        [String]$channel = '#general',
        [Parameter(Mandatory = $false,Position = 3,HelpMessage = 'Slack username')]
        [ValidateNotNullorEmpty()]
        [String]$username = 'PowerShell script',
        [Parameter(Mandatory = $false,Position = 4,HelpMessage = 'Icon emoji')]
        [ValidateNotNullorEmpty()]
        [String]$icon_emoji = ":bomb:"
    )

    Process {

        $payload = [PSCustomObject] @{
            username   = $username
            channel    = $channel
            icon_emoji = $icon_emoji
            text       = $message
        }

        $body = $payload | ConvertTo-Json

        # Call the API
        try 
        {
            Invoke-RestMethod -Uri $uri -Method POST -ContentType "application/json" -Body $body
        }
        catch 
        {
            throw 'Unable to call the API'
        }
    } # End of process
} # End of function

# See https://api.slack.com/apps 
# and https://my.slack.com/services/new/incoming-webhook/