Usually the way to accept a User Agreement into an Online Form is to add a checkbox and asking the user to agree to your terms and conditions or something else. This method is called "Point and Click" and is common practice on the web. The following text explain how to do it with our Web App Forms Builder
The first step is to add in your form your terms & conditions or whatever you’re having the user agree to. To do this, you can use a Text Area Field of our Form Builder. You can either link to your terms & conditions or paste them right into the description area field.

The second step is to add a CheckBox Field to your Online Form. In the question label write the effect of the action like “By checking this box, you agree to our terms & conditions above”. Select the "required" option of the CheckBox Field than the write in the only option of CheckBox Field “I agree”.
Alternately, in the cases whitch you don't must display the legal text, you can avoid to copy/pasting all your legal text into your form simply adding a HTML markup link to your Terms of Service web page in the text of your CheckBox Field choice.

The result is that now the user cannot submit your form without checking that they agree to your terms & conditions?

While we can’t give legal advice to our users, this type of “signature” has been used on the web for years and should serve for most purposes. The "Point and Click" method usually perfectly legal in many country for Person to Business agreement but we suggest to send a confirm by email.