I'm not clear what your problem is. Can you walk us step by step so that we can re-create your problem? here's what I tried and it seems to work ok

I uploaded a random picture that will get deleted after 7 days.
I then pasted the link into an mIRC editbox:

/echo -a https://postimg.cc/LhZG8bK4

I can then rightclick and choose 'copy link' and it's correctly added into the clipboard, including your missing colon.
I can also choose 'open link', and it's sent to the browser including the colon.