FIXED. Verified on 5.10.0.24.20181113213923_03b81fd.

Request: POST /api/service_catalogs/:id/service_templates/:id
Query: {
	"action": "order",
	"resource": {
		"input_dropdown": "One",
		"text_box": "hello"
	}
}
Response: {
	"href": "https://<ip_address>/api/service_requests/3",
	"id": "3",
	"description": "Provisioning Service [catalog_item_1] from [catalog_item_1]",
	"approval_state": "pending_approval",
	"type": "ServiceTemplateProvisionRequest",
	"created_on": "2018-11-20T10:38:11Z",
	"updated_on": "2018-11-20T10:38:11Z",
	"fulfilled_on": null,
	"requester_id": "1",
	"requester_name": "Administrator",
	"request_type": "clone_to_service",
	"request_state": "pending",
	"message": "Service_Template_Provisioning - Request Created",
	"status": "Ok",
	"options": {
		"dialog": {
			"dialog_input_dropdown": "One",
			"dialog_text_box": "hello"
		},
		"workflow_settings": {
			"resource_action_id": "25",
			"dialog_id": "2"
		},
		"initiator": null,
		"src_id": "1",
		"request_options": {
			"submit_workflow": true,
			"init_defaults": false
		},
		"cart_state": "ordered",
		"requester_group": "EvmGroup-super_administrator"
	},
	"userid": "admin",
	"source_id": "1",
	"source_type": "ServiceTemplate",
	"destination_id": null,
	"destination_type": null,
	"tenant_id": "1",
	"service_order_id": "3",
	"process": true,
	"cancelation_status": null,
	"service_templates": {
		"count": 0,
		"resources": [],
		"actions": [
			...
		]
	}
}

And I checked the automation log, static fields are populated with the supplied value.