There isn’t a dedicated “lock” toggle to pin one stop to a fixed position. Instead, you control sequence by arranging stops manually with drag-and-drop, and that order stays until you choose to re-optimize (which recalculates the sequence). To keep a stop first or last, set it as your start or end location; to influence inclusion order, use Priority stops. Applies to both EasyRoutes for Shopify and EasyRoutes for Web.
See: Editing routes · Priority stops