Install and activate Popup Maker.
Go to Popup Maker > Add Popup and create a new popup named say,
Click on Add Form button above the WordPress editor and select your form making sure Ajax is enabled.
In the Popup Settings metabox, add a Click Open trigger.
In the Display section, select Light Box Popup Theme.
In Close > Alternate Methods you might want to tick “Click Overlay to Close” and “Press ESC to Close”.
Publish the Popup.
Edit your button anchor or button tag HTML and add a class of