Every request of this API needs to include a header named X-Liquid-Signature that contains the string output from SHA-256 hash calculation whose input is the timestamp, some indicated fields used in the request and the secret API key that is unique for each integration and will be provided by Liquid Barcodes. It is important to make efforts to obfuscate this key in the app implementation.
Technical details about this signature calculation can be read in this section of the API Reference. Below it, there is a section for every request defining the fields that are used for that calculation.