You may already have some CRM web forms in use that are published in your website. You need to follow the steps given below to add a consent checkbox to those web form. The same steps can also be followed to include consent checkbox in a new form.

Step 1 - Create a Consent/Opt-in field in your CRM account.

First, create a checkbox type field that will store the consent information. 

1. Go to Setup > Customisation > Modules and Fields.
   
2. Click the desired module and choose a layout.
   
3. From the New Fields Tray on the left, drag and drop the checkbox field type into the desired module section on the right.
   
4. Name the field and define the Field Properties as required for the field.
   
5. Click Save.
   

Step 2 - Include that field in your web form and generate the form's code.

Drag and drop fields to build a form and generate the code for the web form. 

1. Go to Setup > Developer Space> Webforms.
   
2. In the Web Forms page, choose the Module from the drop-down list.
   The existing web forms for the selected module will be listed.
3. Click Create Web Form or choose one of the existing forms to edit.
   
4. Under the Fields List tab, click on the checkbox field that you created and drag & drop it in the form builder.
   
5. Save the changes and generate the code.
   
6. Copy the code to publish it in your website.
   

Step 3 - Publish the form in your web page.

The final step is to use the new code for the web form and publish in the website. You can edit the text for the field in the form's HTML code but make sure the  name value is not changed as this value is used to capture data from the web form and pass it on to the CRM.