Hi Abdul Basith Abdul Bari,
Thanks for the response.
While external users upload files are not supported in Forms for now, it could be very hard to collect the signatures. I find one user shared his idea on how to accomplish this with a workaround.
Attach files to a form for outside users - Microsoft Community Hub (see jessb1445's replies)
"What I ended up doing was adding a link to request files to a onedive folder. Then set up a flow that moves that file to a sharepoint folder and notify someone. The sharepoint folder I've moved it to has a lookup field to a list from the form responses. When someone fills in the document field for their name (lookup field), it adds it as an attachment to the item. This is the only workaround that I have found works."
Hope this can help.
Sincerely,
Ryan | Microsoft Community Moderator