In case you are planning to develop for Linnworks as a 3rd party and do not have your own Linnworks account, which automatically also gives you access to the Developer Portal, then you can request a Developer Account following the steps given here.
Whether you already have a Linnworks account with Developer access but need a sandbox environment to test your custom work outside of it, you would need to contact our Sales Team.
In order to do so, please raise a ticket in our Support Centre with Category as Sales and Billing and Sub-category as My Account:
Please provide details on why you would be needing a separate sandbox environment, the Linnworks/Developer Account you are currently working with and your contact information.
Full instructions for raising a Support Ticket are available here.
Our Sales Team agents will then be in contact with you at their earliest availability.