//Create the valueSet for picklist type
MetadataService.ValueSet picklistValueSet = new MetadataService.ValueSet();

//For each ValueSet, we have either ValueSetValuesDefinition or ValueSettings and some other attributes
MetadataService.ValueSetValuesDefinition valueDefinition = new MetadataService.ValueSetValuesDefinition();

List<MetadataService.CustomValue> values = new List<MetadataService.CustomValue>();
MetadataService.CustomValue customValue1 = new MetadataService.CustomValue();
    //Adding "Default" value as one picklist value in the newly created picklist
  customValue1.fullName = valFullName ;
  customValue1.description = '';
  customValue1.isActive =TRUE;
  customValue1.default_x = FALSE;
  customValue1.label = valLabel;
  values.add(customValue1);

//It will be list of CustomValue
valueDefinition.value = values;
valueDefinition.sorted = false;

//set the valueSetDefinition
picklistValueSet.valueSetDefinition = valueDefinition;

//Set the valueSet for picklist type
customField.valueSet = picklistValueSet;