Hi,
Do you mean masking the URL on the form itself, or in a view on a report/dashboard?
If it's on the form itself, you could use a descriptive text field type. With this type of field you can choose what text you want to appear and embed a URL that is clickable for the end user.
If you're wanting to display the URL in a view, you can use the following text mode in a column. Just replace the OBJECT TYPE (i.e. "project") and custom field name as required. The URL within the custom field will then be displayed as "Click Here" in your view.
displayname=Embedded Hyperlink
link.url=customDataLabelsAsString(OBJECT TYPE.FIELD NAME)
linkedname=html(URL)
textmode=true
valueexpression=CONCAT("Click Here")
valueformat=HTML
Hope that helps.
Regards,
Rich.