The Hidden Managed Metadata FieldThe trick to being able to save a taxonomy field value via REST, is knowing the name of the hidden text field that gets created when you create a Managed Metadata field.
When you make a REST call to update the Managed Metadata field, you need to pass a string representation of the managed metadata terms that you're saving into this hidden field.
The string representation of the terms is in the format of "termName|termId;".
In the screen shot below, you can see the field value of a hidden Managed Metadata field (the hidden field name is pa28754972f84ff7a6bf3e4762f23e23).
Finding the Name of the Hidden FieldFinding the name of the hidden text field of a Managed Metadata field is easy. The fastest way to do it, is by using the REST API via a browser.
1. Open Internet Explorer
2. In the address bar, enter the URL for your Office 365, followed by the REST API endpoint for to list the fields in your list.
3. Use the browsers find function (Ctrl+F) to locate your field name in the xml
4. Look at the TextField property, and copy the copy
5. Use the browsers find function (Ctrl+F) to locate your field name in the xml, by searching for the guid
6. Make note of the hidden fields static name
Formatting a taxonomy field valueNow that you know the name of the hdden Taxonomy text field, you need to format the term values you want to save.
The terms should be formatted in the format of "termName|termId;".
An example model for updating the list in the above screens could look like this: