Why does the From email field not allow you to select a pre-defined 'Hide My Email' email address?

Hi All, 

I have set up a  'Hide My Email'* email address for a company. I use this to log in on-line and via their app. I have also given that email to their Customer Services to use and we've been engaging successfully. So they write to me on e.g. 'random_emai1@icloud.com' and I reply. 

I'm assuming this part of the process works fine as I'm receiving the emails and I can see the random_emai1@icloud.com in the 'To' field. 

My query is I now want to initiate the email to said company, however the mail app (trying to be helpful) locks down the 'From' field and only provides my main 'original' email address and an option to 'create' a new hidden email. Is this by design? Am I miss using this function? 

PS Generating a 'new' hidden email will just confuse matters. I just want ALL my comms with each company to flow (hidden) to a predefined account and later resolve to my actual account. 

*E.g. random_emai1@icloud.com (PS NOT A REAL EMAIL)

Thanks M