It seems like you may be mixing API versions.
Version api-version=2018-06-01-preview is deprecated, and open api is not an available option is this version. It uses a content type header of "application/vnd.swagger.doc+json" instead of the "format": "swagger-link-json" property.
Updating to api-version 2019-01-01 allowed me to import the OpenAPI spec. The APIM management base and authentication stayed the same, but the rest of the format needs to match the standard Azure Management API style https://(resourcename}.management.azure-api.net/subscriptions/{subscriptionid}/resourceGroups/{resourcegroup}/providers/Microsoft.ApiManagement/service/(resourcename}/apis/petstore?api-version=2019-01-01
From there the format parameter works as expected for both Swagger and OpenAPI;
{
"properties": {
"format": "openapi+json-link",
"value": "http://petstore.swagger.io/v2/swagger.json",
"path": "petstore"
}
}