Set default value to taxonomy field
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Term defaultValue = terms[0];
TaxonomyField taxField
//Check and get the field value if already default value
TaxonomyFieldValue fieldValue = (TaxonomyFieldValue)taxField.DefaultValueTyped;
//Other use this
TaxonomyFieldValue fieldValue = new TaxonomyFieldValue(Field);
//Compose the fieldvalue (label | termguid)
string text = defaultValue.Labels[0].Value + TaxonomyField.TaxonomyGuidLabelDelimiter + defaultValue.Id.ToString();
fieldValue.PopulateFromLabelGuidPair(text);
//Check if the taxonomy term is not allready in the TaxonomyHiddenList of the rootsitecollection
int[] ids = TaxonomyField.GetWssIdsOfTerm(web.Site, store.Id, productenTermSet.Id, defaultValue.Id, true, 1);
if (ids.Length == 0)
{
//Set the WssId (TaxonomyHiddenList ID) to -1 so that it is added to the TaxonomyHiddenList
fieldValue.WssId = -1;
}
//Set the default value
taxField.DefaultValue = fieldValue.ValidatedString;
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