Back at Gridbox Editor, drag and drop Calendly component from Components Panel -> Integrations
Change the button text if you want to. (Click on the button and click on </> icon from the blue highlighted toolbar -> change the text and save)
Click on settings icon from highlighted toolbar wrapped around the selected button or open the properties panel from the left sidebar.
Enter the Calendly URL, which you can find in My Calendly page, as mentioned in the image below