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Special Notes on Adding Options to Tag and Dropdown Fields
If you're adding options to tag/dropdown fields, be careful not to overwrite the other options already present by using the addOnly parameter.
Note also that tags and dropdowns take slightly different names for their data (tagSettings v dropdownSettings, tags v items, tag v name). Make sure to use the correct one.
Take a look at the following examples to guide you through the details - the first example is for a tag field and the second is for a dropdown field.

curl POST 'https://api.streak.com/api/v1/pipelines/pipelineKey/fields/fieldKey?addOnly=true'  -H 'content-type: application/json'  --data '{"tagSettings":{"tags":[{"tag":"Meow"},{"tag":"Woof"}]}}' -u YOUR_API_KEY:

curl POST 'https://api.streak.com/api/v1/pipelines/pipelineKey/fields/fieldKey?addOnly=true'  -H 'content-type: application/json'  --data '{"dropdownSettings":{"items":[{"name":"Meow"},{"name":"Woof"}]}}' -u YOUR_API_KEY:

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Content Type and Auth
Make sure to include -H "Content-Type: application/json" and your authorization in any request you make on your own.