If you're referring to uploading a file using a web form, you have to know what the web end-point is (i.e. the part that is processing the form).
The fact that you're getting a webpage indicates that you are trying to upload a file to a form. Without additional information, it would be next to impossible to help due to the sheer number of possibilities of how this can be implemented in the back end.
If you are uploading a file to a server that you do not control, it's best to get in touch with the web developer/host to find out if there is an API that you can use to upload files. The API end-point can then be used by curl to send the file.
If this is a server you do control, I would recommend either scp, sftp, or ftp to upload the file instead of going through a form.