Public/New-TrelloCustomFieldOption.ps1

function New-TrelloCustomFieldOption {
    [CmdletBinding()]
    param
    (
        [Parameter(Mandatory, ValueFromPipeline)]
        [ValidateNotNullOrEmpty()]
        [pscustomobject]$CustomField,

        [Parameter(Mandatory)]
        [string[]]$Value
    )

    $ErrorActionPreference = 'Stop'

    $RestParams = @{
        Method      = 'POST'
        ContentType = 'application/json'
    }

    if ('options' -in $CustomField.PSObject.Properties.Name) {
        $restParams.Uri = '{0}/customField/{1}/options?{2}' -f $script:baseUrl, $CustomField.Id, $trelloConfig.String
        foreach ($val in $Value) {
            $restParams.Body = (ConvertTo-Json @{ 'value' = @{ 'text' = $val } })

            $null = Invoke-RestMethod @RestParams
        }
    } else {
        throw 'Custom field does not support options.'
    }
}