You can set the configuration for dynamic content to display manual product recommendations that you decide upon and that aren't generated by any other category or parameters.
Access manual product recommendation settings by going to STORES > Configuration> Dotdigital > Dynamic Content.
The products that you wish to manually push need to be entered as comma separated product IDs under 'Products'.
It's useful to have fallback products ready should any products due to go into your emails be unusable, thus they can be replaced. Once again, these fallbacks need to be entered as comma separated product IDs under 'Products'.
Click Save Config after making any changes.