When you open URL usually you can choose a field within your dataset that contains the URL (if it is a list details block). Is it:
a) a solution on your list details block to construct the URL in your dataset with all params required?
b) if it is, Passing URL Parameters Through a Softr Form this post talks about the possibility of passing tokens in the open external URL action and how its not a feature yet. +1 on this request 
for perhaps a different approach? something like : www.myrandomwebsite.com/?param1={Value}