How to replace "space" in string to "%20"

I was building the attachment URL as per example on

Nintex Workflow - Get Attachment Urls UDA - Vadim Tabakman 

I successfully build the string of the URL.

But, the problem occur, when the attachment name have multiple space. e.g.: "abc<space><space>def.txt"

This will cause generated URL "https:\blahblahblahlalalaabc  def.txt" broken or unable access by the broswer.

The proper URL should be "https:\blahblahblahlalalaabc%20%20def.txt".

How can I replace the <space> to %20 in O365 nintex workflow string? Since you can't put <space> in the replace string action?

Any idea?