Server File Upload to webhost

I bought a flutter source code from Codecanyon that uses dart apps than need connection to backend server files to function. The author of the code gave a scanty documentation on installation and on how to connect the apps to the backend and he is not available. I have managed to install the Dart app but I cannot connect it to the server backend. Am looking for someone with a lot of experience to help connect the app to the backend. I have the backend server files and I have already uploaded them to my webhost but I cant connect the two. I have the documentation with the installation instructions from the author but they are very scanty and insufficient .I have attached the documentation from the author