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 app.paymash.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!