the B4XCombobox makes use of Spinner which from what I gather does not have a Font method.
Try this code which uses the CSBuilder . Make sure to copy your font to the assets folder.
B4X:
lstItems.AddAll(Array As String("Dog", "Cat", "Bird"))
Dim cs As CSBuilder
Dim myFont As Typeface = Typeface.LoadFromAssets("MyFont.ttf")
For i = 0 To lstItems.Size -1
B4XComboBox1.cmbBox.AddAll(Array(cs.Initialize.Typeface(myFont).Append(lstItems.Get(i)).PopAll))
Next
mj's code works very well, but if you do not want to expose the native spinner (cmbBox), you can use this code as an alternative
B4X:
Dim l2 As List
l2.Initialize
For i =0 To lstItems.Size -1
l2.Add(cs.Initialize.Typeface(myFont).Append(lstItems.Get(i)).PopAll)
Next
B4XComboBox1.SetItems(l2)