I ran into this exact same problem and figured out the solution: Basically, you can't create a connector from "Blank". You need to use an OpenAPI file (aka Swagger file) to create the connector. I found a large part of the answer in this post:
http://www.cleverworkarounds.com/2017/11/13/a-sample-openapiswagger-file-for-powerapps/
But the swagger file example included there only includes name/value pairs in the POST request, which doesn't help when you need to include JSON. To correct it, I added the body parameter as showin in the complete OpenAPI file here:
{
"swagger": "2.0",
"info": {
"description": "Send me an email notification",
"version": "1.0.0",
"title": "SendEmail"
},
"host": "[enter host here].logic.azure.com",
"basePath": "/workflows",
"schemes": [
"https"
],
"consumes": [
"application/json"
],
"produces": [],
"paths": {
"/[enterid here]/triggers/manual/paths/invoke": {
"post": {
"summary": "Call my flow",
"description": "Call my flow desc",
"operationId": "CallMyFlow",
"consumes": [
"multipart/form-data",
"application/json"
],
"parameters": [
{
"name": "api-version",
"in": "query",
"default": "2016-06-01",
"required": true,
"x-ms-visibility": "internal",
"type": "string"
},
{
"name": "sp",
"in": "query",
"default": "/triggers/manual/run",
"required": true,
"x-ms-visibility": "internal",
"type": "string"
},
{
"name": "sv",
"in": "query",
"default": "1.0",
"required": true,
"x-ms-visibility": "internal",
"type": "string"
},
{
"name": "sig",
"in": "query",
"default": "[enter sig here]",
"required": true,
"x-ms-visibility": "internal",
"type": "string"
},
{
"name": "body",
"in": "body",
"required": true,
"x-ms-visibility": "internal",
"schema":
{
"type": "object",
"properties" :
{
"fruit":
{
"type":"string",
"example":"Apple"
},
"size":
{
"type":"string",
"example":"large"
},
"color":
{
"type":"string",
"example":"Red"
}
}
}
}
],
"responses": {
"200": {
"description": "successful operation"
}
}
}
}
},
"definitions": {},
"parameters": {},
"responses": {},
"securityDefinitions": {},
"security": [],
"tags": []
}
