It's possible to offer add-on products as upsell in two ways:

To deploy Upsell in MyOrder, you need to create an additional sales channel. This will be the shop where the fan is directed from MyOrder to place a new order. Here's how to do tha
If you've never set up upselling before, go to 'Create' and navigate to 'Sales Channels'.
Create a new sales channel for Upsell. Choose Type: 'Online (shop)'.
Go to 'Events' and then 'Sales Configuration'. Click the pink button 'Create New Sale' and choose the sales channel you just created.
Now add the desired additional products by clicking on 'Add Products to Sale'. Then save with the green button 'Apply Changes'.
Don't forget to publish the Upsell shop by sliding the slider from red to green.
Go to 'Events', click on your desired event and select 'Sales Configuration'.
Click on the sales channel you want to setup Upsell for.
Within the sales channel, click on the black 'Edit' button on the right side.
Expand the 'Upselling' section.
Check the 'Enable upsell in MyOrder' box. Add the sales channel you created for Upsell.
Enter '5' under 'Number of recommended products' and add the add-ons.
If you want to show product images in the upsell shop, check these checkboxes.
Don't forget to save your changes at the bottom.
Double-check if you see the add-ons appear in the shop at checkout. It should look like this: