There is no "escape character" in QlikView so far, but you can use an action in your button, Select in Field, and use the pattern "?A*". That of course will select all values where A is preceded by one sign, irrespective of what this character is, but it might do.
Hope that helps.
Thanks for your help!!
I have already used this pattern ("?A*"). It also works but the problem is that there are also data for example "Bau" that get selected with this pattern but they shouldn't because they don't start with an "=" sign.
Is there any possibility to use the "=" sign like a String?
I tried the funktion: =Text('=A*'). But it don't work!