Go "Bots" -> "WhatsApp" then "360Dialog" on NicoChat and in your WhatsApp App, add "+4930609859535" as a new contact and sent "START" to it.
Copy the "api-key" back to NicoChat, click "Add WhatsApp Number":
Again put the number "+4930609859535" here and paste the API key. After adding this number you can click the "Create Bot" to start building a flow.
To use 360Dialog sandbox, please note that:
only the phone number which requests the API key can test the bot
you can send "START" to get a new code, but make sure you update the key with your bot.
360Dialog doesn't allow sandbox users to access their media recourses. For example, if you use a question step to ask for a photo and store the URL in a text variable. You are unable to visit this URL to get the photo because you are using a sandbox.