XFM PBA definition: type expression, character '='

Hi,

my PBA is defined like this (simplified):

<PBA name="Doplnkovy_popis" type="expression">[System.String.Trim("aaa=bbb ")]</PBA>

there is a problem with '=' character, it seems like this character is reserved for expression purposes.

Any idea/workaround how to support  =  character in string parameter?

thanks in advance

Jan