You can supply a custom controller configuration file on the root of any FAT32 connected file system.
You can follow the guide outlined on the official website.
https://retrogames.biz/support/thespectrum/controller-configuration/
Once you have your file named gamecontrollerdb.txt ready to go in the root of the connected USB drive, you'll want to add the following information to the file depending on the adapter version you are using.
03000000eb030000f42f000011010000,Monster Joysticks Ltd. DE9 to USB V2,a:b0,b:b1,back:b6,leftx:a0,lefty:a1,leftshoulder:b4,rightshoulder:b5,start:b7,x:b3,y:b2,platform:Linux,
03000000c0160000dc27000001010000,Monster Joysticks MJ2DB9,a:b1,b:b4,back:b6,leftx:a0,lefty:a1,leftshoulder:b0,rightshoulder:b5,start:b7,x:b3,y:b2,platform:Linux,