To add a video to the online store you first need the appropriate Paymash license and you have to activate the online store. You can find more information here.
Log in to https://app.mypaymash.com/ and go to "Webshop".
Choose an already created page or create a new one
In the page click on "Choose Layout" and select a layout
Then select "Add Video"
Now you will be asked to add a Youtube or Vimeo link. Attention: Shortened URL's, e.g. from bit.ly, do not work.
Save your input and you have added a video to your online store!