If Windows was installed using the Boot Camp Assistant, then neither Apple nor Microsoft offer a practical way to increase the Windows partition, other than remove and reinstall Windows.
The Boot Camp Assistant will only install Windows 10 on the newer Mac models. If you have an older model, the Boot Camp Assistant may still be able to install an older version of Windows. Afterwards, you may be able to upgrade to Windows 10.
If Windows 10 is activated, then you do not need the product key to reinstall or upgrade. However, if you first reinstalled an older version of Windows (XP, Vista, 7, 8 or 8.1), you may need the product key for the older version.
In your question, you stated you partitioned. If this was successful and Windows does not boot, then you probably have Windows installed to BIOS boot. An effort could be made to try an make Windows bootable. There may be no point in trying, if you are going to reinstall Windows anyway. This a decision you will have to make.
If you have an older model Mac and the Boot Camp Assistant will not let you install Windows 10, then it is usually possible to install Windows 10 without using the Boot Camp Assistant. The instruction depend on your model Mac.
I should mention there are third party tools that can aid in enlarging the Windows partition without the loss of data. WinClone is one such product.
Comment about the Linked Video
The video is a deliberate attempt to deceive the viewer. At the time the video was made the Boot Camp Assistant only installed BIOS booting Windows. This required using hybrid partitioning. The MiniTool Partitioning Wizard does not properly handle hybrid partitioned drives. Specifically, the Wizard correctly updates the MBR partition table, but fails to update the Guid Partition Table (GPT). The video is fraudulent and anyone with experience can easily see this. At time mark 1:46, free space is created between the OS X partitions and the Windows partition. At time mark 2:40, the free space can be viewed as occurring after the Windows partition. So, at some point not shown in the video, changes were made. At time mark 3:15, the Windows partition is extended. In fact, the Windows partition would also need to be moved to be beginning of the free space created at time mark 1:46.
Comment about the Linked Question
I would advise not to follow any answers to a question that were not accepted by the OP and have comments that indicate none of answers worked.