Applying a Character Format to a Character

I am trying to apply a character format to a single character.

I have tried :

MyDoc.SetTextVal(myrange,Constants.FO_CharFmt ,"RoundFont")

and

var charFmtId = MyDoc.GetNamedObject(Constants.FO_CharFmt, "RoundFont");

var myprops = MyDoc.GetProps(charFmtId)

MyDoc.SetTextProps(myrange, myprops); 

neither work unfortunately - I have been trying for two weeks!

I know the "myrange" is Ok because I can delete characters in the range correctly and add a character instead.