Fast Fonts responds to the fast-font-selector: URL scheme, with the following options:

Option Abbreviation Parameters Explanation
callback cb <URL> URL scheme to call in order to return chosen font
magic   <text> Arbitrary characters to pass back to caller with result
welcomeTitle wt <title> Title to display in AlertView on launch
welcomeMessage wm <text> Text to display in AlertView on launch
welcomeDismiss wd <label> Button text used to dismiss AlertView
confirmChoiceText ct <label> Text for confirmation button
fontFamilies f <family>[,family]* Comma-separated list of font families to offer
fontNames n <name>[,name]* Comma-separated list of font names to offer
fontSizes sz <size>[,size]* Comma-separated list of font sizes to offer
fontVariants var <name>[,name]* Comma-separated list of variants (e.g. Bold, Italic, BoldItalic) to offer
defaultFont df <font> Font to initially select
defaultSize ds <size> Size to initially select
displayText txt <text> Text to display in lower screen area
readonly ro [boolean¦(readonly¦read)¦(readwrite¦write¦writable)] Whether displayText can be modified
debug   [boolean¦(save¦keep)¦(clear¦unset)] Whether to enable debug code and options

... where boolean is a value matching <(0¦off¦false)¦(1¦on¦true)>.

e.g. fast-font-selector:callback=<private scheme: or URL>;magic=test;welcomeTitle=Fast%20Fonts%20Test;welcomeMessage=API%20Test;welcomeDismiss=Continue;confirmChoiceText=Select%20This%20Font;displayText=Example%20text

When the button with title 'confirmChoiceText' is tapped in Fast Fonts, the calling application is called by the specified scheme with parameters 'font=' followed by the Font Name, 'size=' followed by the Font Size, 'text=' followed by the contents of the lower text-area (if not read-only), and 'magic' (if passed in originally).