I have found a workaround for this.
(I think you must have a business account, I don't think this will work with a personal account, the request files option is not there on personal accounts)
Create a folder in OneDrive
On the folder select the three dots (more options) button
Select Request Files (make sure you allow anyone outside the organization to upload)
Copy the link provided by the request files option
Insert the link into the form that you have made (make sure you allow anyone to fill out the form)
This has worked for me.
The good thing is, the people have access to upload the files. It asks for their name when they upload, so you can track who uploaded what (it adds their name to the filename). They do not have visibility to see what anyone else has uploaded.