@BrunoSousa1 The reason, to put it simply, is that the copy & paste function does not only evlove the strings/codes themselves that we can see from the UI but also needs to support the data source structure behind it if evolved, e.g. getting the field id from multiple data sources, etc.
We are working on these kinds of enhancements to provide a better user experience in the future. Thanks for staying with us.
Another little snag is that all the text displayed by esri is sans serif. So me typing in "...jRf8jjFwxedITdFe/arcgis/rest/services..." (or whatever address) becomes annoying when that l is really an I, or vice versa.
Btw, a friend of mine found a workaround for pasting strings into the expression. He just opened developer tools on the browser and found the html for the expression box and pasted the URL there. The pasted text shows up in the UI and you can save it from there.