Customized Term Set option getting disabled after creating Managed Metadata site column through script

Hi,

I am creating Managed Metadata site column through power shell script as below

Now problem is ‘Customized Term Set’ option is getting disabled after creating Managed Metadata site column through code.

Please let me know If there is any way to enable the ‘Customized term set’option for the created Managed Metadata site column?

$wikiurl = “http://abc.com”

web = get-spweb $wikiurl

#----------Region to create Site Coulmns---------

#Creating Metadata column

$fieldName = “MMSColumn1”

#Open the Site + Web

$site = Get-SPSite $wikiurl

$web = $site.RootWeb

#Gets a TaxonomySession object

$taxonomy = Get-SPTaxonomySession -Site $wikiurl

#Gets the GUID of the MMS Application

$sspId = $taxonomy.TermStores |

    Where-Object {$_.Name -eq “mmsappname”} |

    Select-Object -ExpandProperty Id |

    Select-Object -ExpandProperty Guid

#Gets the GUID of the Term Set

$termSetId = $taxonomy.TermStores.Groups |

    Where-Object {$_.Name -eq “mmsgname”} |

    Select-Object -ExpandProperty TermSets |

    Where-Object {$_.Name -eq “mmstsetname”} |

    Select-Object -ExpandProperty Id |

    Select-Object -ExpandProperty Guid

#Creates a Field

$field = $web.Fields.CreateNewField("TaxonomyFieldType",$fieldName)

$web.fields.add($field)

#Updates with Term Set + SspId

$field = $web.fields.GetField($fieldName)

##$field.TermSetId = $termSetId

$field.SspId =  $sspId

$field.Required = $true

$field.Update()

$fieldLink = new-object Microsoft.SharePoint.SPFieldLink($field)

$ctype.fieldlinks.add($fieldLink)

$ctype.update()

}

Thanks & Regards, Sudheer