How can I add a check constraint for JSON in Entity Framework?

I think that EF don't support any kind of CHECK constraints. The only thing that you can use is migration. See example in: Is it possible to add CHECK constraint with fluent API in EF7?

As per the accepted answer, you'll need to add a migration. This is the EF Core syntax:

    protected override void Up(MigrationBuilder migrationBuilder)
    {
        migrationBuilder.Sql("ALTER TABLE dbo.Data ADD CONSTRAINT CK_Data_JsonData_MustBeJson CHECK (IsJson(JsonData) = 1);");
    }

    protected override void Down(MigrationBuilder migrationBuilder)
    {
        migrationBuilder.Sql("ALTER TABLE dbo.Applications DROP CONSTRAINT CK_Data_JsonData_MustBeJson;");
    }