While searching for the Mailto String formatting, I found this tool.
Mailto | The mailto encoder
Encode full emails as a mailto. We do the hard work to url encode your subject and body for emails with special characters and emojis. Just paste the result in your html anchor element!
mailto.now.sh
B4X:
Dim mMailAdress As String = "?" '"xxx@xxx.de"
Dim mMailSubject As String = "Einkauf" & "%20Real" 'Leerzeichen = %20
Dim mMailBody As String = "xxx%0A%0Aaaa%0A%0Abbb" 'Zeilenumbruch, 1x = %0A, 2x %0A%0A
Fx.ShowExternalDocument("mailto:" & mMailAdress & "subject=" & mMailSubject & "&body=" & mMailBody)