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 web page using this URL. FileMaker Pro uses the following URL encoding rules to fix only the most common problems:
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ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
0123456789-_.~!*'();:@=+$,/?[]
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Notes
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Because FileMaker Pro cannot correct all possible URL encoding problems, test your web viewer to ensure that its URLs are correctly encoded.
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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 
Defining a custom web address
Troubleshooting layouts with web viewers