online reply form not sent

Change your default web browser.  I''ll bet it is a url instead of a mailto link.

the mail forms adress  is :  name@company.be
i was thinking that i had to change the default mail program somewhere in windows - control panel?
i would like to change the view of control panel but do'nt see any options to click on   ( see image)

You said "everytime i have an online form to reply to a website, it opens edge" which tells me the link to the form is a URL and the default browser in 10 is Edge.  To get the Control Panel on a Windows 10 PC you have to go to Settings -> Personalization -> Themes -> Change Desktop Icons and I generally check all of them.  You could also get to the default App settings by clicking the Apps link in the Settings screen you posted.

yes, the default browser is edge - but not the default mail app
in themes i see the desktop icons, but that's not what i want to change, i want the 'small icons' view, as i get when:
in search i type control panel, then i choose small icons
i cannot find that when i open directly the settings screen - icon above start

It is not the same thing!  Windows 10 wants you to use the settings app; but there are several things that are much easier and faster still in the control panel and all you can do is to show it on the desktop in themes.

does changing the desktop icons cheange the view of CP too?
anyway  -let's return to the reply form not sent - how to fix that?

No it does not change the view; but, the view is remembered if you change it once (and I have personally done just that, thousands of times).
In your original posting, you said that the form opened in Edge which means that it is a URL and not a mailto entry.  The only way to fix a mailto link is to have a properly working local mail app; however, that will still not cause a URL to open in your mail program nor would you want it to.
Are we talking about a link in an email or on a webpage?

maybe i should rephrase the Q :
i fill  the online form - then click send
all is well - 1/2 minute later it informs me it could NOT send the mail

when i click on that message, i see it opens theis window ( i thought it was edge, but it seems to be the isp mail app, see pic
i suppose if i could get it to use my default mail, all is well

Control Panel -> Default Programs -> Mail and Change it to Thunderbird.  That's the Windows 10 Mail App as a note and it stinks.
The failed delivery notice; however, is most probably because the website isn't working correctly; online forms can be fairly tricky.

ok i have changed the default mail ; in essence that was what i was looking for. i hope it works ok now

i could test it again and it still does not open thunderbird