Step 1:

Click the settings icon from the top right of the checklist.

Step 2:

Select the Opt-In Form from the side menu, and set Heading and Sub-Heading for your Opt-IN Form, You can write support text, can set an image for your Opt-In Form. and can write the text for the submit button.

Step 3:

And click the save button at the bottom to apply the changes.