executing workflow with REST

I've made this call from a python script using requests.post and I get a 200 return status and a list of the executions (just like I get when I do a requests.get with the same url).

Inputs:

params = {'workflowId': 'a81f071d-8bd0-4f94-a4b0-8af4c030d0b6'}

data = {'parameters': [{'name': 'criteria', 'type': 'string', 'value': {'string': {'value': 'BLUEDC12A'}}}]}

URL

https://vm-prd01vro:8281/vco/api/workflows/a81f071d-8bd0-4f94-a4b0-8af4c030d0b6/executions?workflowId=a81f071d-8bd0-4f94-a4b0-8af4c030d0b6

Return:

Status=200

Reponse={'relations': {'total': 1, 'link': [{'href': 'https://vm-prd01vro:8281/vco/api/workflows/a81f071d-8bd0-4f94-a4b0-8af4c030d0b6/', 'rel': 'up'}, {'href': 'https://vm-prd01vro:8281/vco/api/workflows/a81f071d-8bd0-4f94-a4b0-8af4c030d0b6/executions/', 'rel': 'add'}, {'attributes': [{'value': '2c91e4a35b8cb581015b8d0b774a0101', 'name': 'id'}, {'value': '2017-04-20T16:26:30.603-04:00', 'name': 'startDate'}, {'value': '2017-04-20T16:26:32.627-04:00', 'name': 'endDate'}, {'value': 'd104647@corp.unifirst.com', 'name': 'startedBy'}, {'value': 'completed', 'name': 'state'}, {'value': 'unf_Get virtual machines by name', 'name': 'name'}, {'name': 'currentItemDisplayName'}], 'href': 'https://vm-prd01vro:8281/vco/api/workflows/a81f071d-8bd0-4f94-a4b0-8af4c030d0b6/executions/2c91e4a35b8cb581015b8d0b774a0101/', 'rel': 'down'}]}}

I have a feeling that my request body is not correct.  What is the correct format of the body?  Where is this documented?  Is this where I specify input parameters for the workflow?

Thanks for any help

Dom