Hi Gabriel,
Greetings from Syncfusion.
In our current implementation of SfAutoComplete, we can provide only FontFamily name as "Fontname.ttf". Hence to provide “The Custom Font declaration as like in Xamarin controls” we have added it to our feature request list. We will implement this feature and include this in our upcoming 2019 Volume 1 release which will be released by mid of March 2019. You can track the feature details from below link.
Regards,
Dhanasekar