Hmm ... I am seeing the following:
1. I took and epub in Sigil, fired up PageEdit using the external editor button.
2. Selected some text that will become the link text
3. Hit the chain link icon to create the link
4. Did not select anything in the table and instead in the target I typed:
https://www.mobilreread.com
and it worked but if I instead type:
https://www.mobileread.com/forums
I get a dialog message that you can not insert a link at this point. So literally external links are not being properly handled.
I will look into it.