Hey Andrew, welcome to the forums. For this I suggest using the preceding/following inspectors as @jgstew shows here.
In your case though, it does get tricky since actionscript tries to evaluate text inside curly braces as relevance. So I use the percent encoded values %7B and %7D for { and } respectively.
And since the preceding/following only gets the text between the curly braces, I add them back:
parameter "UninstallGuid" = "{"%7B"}{preceding texts of firsts "%7D" of following texts of firsts "%7B" of (parameter "ustring")}{"%7D"}"
You can also escape the curly braces by doubling them up, but in this case I feel it makes things even more confusing especially at the end:
parameter "UninstallGuid" = "{{{preceding texts of firsts "%7D" of following texts of firsts "%7B" of (parameter "ustring")}{"}}"}"
So in retrospect, pretty darn confusing. There is probably a more readable way to do this, but at least if no one else replies these should work.