First you must create a Shotstack account (It is free to create an account)
Here is a link to their site:
https://shotstack.io/

Once you've created an account, take note of the API Keys they provide you.  One API Key is for "Staging" which is to test out your application.  You can use your Staging API Key as much as you want without impacting your Shotstack credits, however they will add their own watermark image to the video as well.  The other API Key is for "Production" which is meant to be used when you deploy your application to be live.

When you've decided which API Key you want to use, provide it to the "Shotstack API Key" field.  If you want to use the Staging API Key, then also check the "Test Mode (Staging)" checkbox.  If you want to use the Production API Key, then make sure the "Test Mode (Staging)" checkbox is not checked.

"Add Watermark To Video" Action's fields:
* Video  -  The video you want to add the watermark to
* Watermark Image  -  The image file you want to use as the watermark
* Shotstack API Key  -  The API Key from your Shotstack account (It can be either Staging or Production)
* Test Mode (Staging)  -  If you are using the Staging API Key, then make sure this box is checked.  If you are using the Production API Key, then make sure this is not checked.
* Watermark Location  -  The location that you want to place your watermark image on the video.
* Horizontal Offset  -  How far horizontally you want the watermark image to be moved from whatever location you chose for the watermark.
* Vertical Offset  -  How far vertically you want the watermark image to be moved from whatever location you chose for the watermark.
* Watermark Opacity  -  The opacity of the watermark image.  It essentially determines how "see-through" the watermark image is.
* Video Resolution  -  The resolution quality of the rendered video with the watermark.
* Scale Image  -  Scale the Watermark Image to be smaller or larger
* Video Length  -  How long the new watermarked video will be

This Action returns 3 fields:
1.  Watermarked Video Id  -  The Video ID to use when fetching the new watermarked Video
2.  Error Occurred   -  If an Error Occurred during the processing of the video
3.  Error Message  -  The Error Text describing what error occurred

"Get Watermarked Video URL" Action's fields:
* Video ID  -  The Video ID returned by the "Add Watermark To Video" Action
* Shotstack API Key  -  The API Key from your Shotstack account (It can be either Staging or Production)
* Test Mode (Staging)  -  If you are using the Staging API Key, then make sure this box is checked.  If you are using the Production API Key, then make sure this is not checked.

This Action returns 3 fields:
1.  URL  -  The URL for the new watermarked video.
2.  Error Occurred   -  If an Error Occurred during the processing of the video
3.  Error Message  -  The Error Text describing what error occurred
4.  Rendering Finished  -  If the video is finished being processed yet.  If this is set to "No", you should wait a few seconds and try again.