About URL encoding in web viewers
FileMaker Pro evaluates a web viewer's web address, encodes the URL by default, then tries to load the webpage using this URL. FileMaker Pro uses the following URL encoding rules to fix only the most common problems:
The following characters are never automatically encoded:
An ampersand (&) is encoded only if a space follows it. For example, "& "is encoded, but "&x" is not.
A pound sign (#) is encoded only if a number character (0 through 9) follows it.
The backslash (\) and percent (%) characters are always encoded.
All other characters are always encoded.
All of the characters listed above are Unicode characters. When a character is encoded, both bytes are encoded separately (unless the first byte is 00), as described by the URL encoding standard. See http://www.w3.org/Addressing/.
Because FileMaker Pro cannot correct all possible URL encoding problems, test your web viewer to ensure that its URLs are correctly encoded.
You can prevent FileMaker Pro from automatically encoding a URL by deselecting the Automatically encode URL checkbox in the Web Viewer Setup dialog box. When this option is deselected, the URL remains as it is entered.
Related topics 
Working with web viewers on layouts