A pre filled webform is often used for agreements, it's important to ensure that the form template is built to accomodate the flow of your form. 

When building a form that you will be pre filling with information before sending the link, make sure that any fields that you don't want the third party to edit are locked.

Forms > View Form Template (of when you are building the form template) > Edit > Form Builder > Desired Field > Properties > Locked for web form