Add white grape juice, strawberries, lemon juice, and salt to a large pot over high heat and bring to a boil.
Once boiling, gently mash the strawberries using a potato masher, then reduce heat to medium-low and simmer for 10 minutes.
Carefully strain the mixture to remove the strawberry pieces.
Let the base cool completely before using, and pour into an airtight container and refrigerate for up to 3 weeks.