How to add (feat.) to a spotify title when that artist is marked as a second primary artist?

I have a question: Is it possible (and how) to add (feat. artist) to the title if the artist whose name is to be in brackets is to be the second primary artist? (in spotify)

what I want to achieve:

                     Artist1, Artist2 - Title (feat. Artist2)

Please show us REAL examples.

Where do you have saved your current metadata? In the Filename? Already in your Tags?
Which Tags do you use?

Do you speak about a Web Source Script? If yes, which one exactly?

For now, I only have a music file without any metadata (sorry, I'm new to this) I would like Artist2 to be included in the title as a guest, but to be listed as the primary artist in the song specification (on spotify)

Your musicfile has no filename?
If you load your music file into Mp3tag and then press Alt+T to show the Extended Tags, do you see only a completely empty popup window?

And please keep in mind what @ohrenkino already told you 9 hours before:

If you release your own music then you have to ask the platform on which you release it about how they want to have it.

yes, my field is empty, I don't have any tags, what should I do?

I think the whole process is controlled by Spotify. If you're an artist, I think you can edit your release metadata via your Spotify for Artists page (I haven't tried this and seen such a page personally!).

There, according to the music metadata guidelines, you can specify different roles for artists, e.g., main artists, featured artists, remixers, composers, lyricists, and producers.

so how do I add a second main artist to the title? Should I fill in some tags or maybe there is another way?

Have you tried to fill in some tags and then see what spotify makes of it?
If spotify does not care, you would have to ask their support on what to do.
MP3tag is a program for actual tagging but does not deal with the philosophy of metadata in files.
If you know what should be done with your files then MP3tag can help you in many cases.

so for Spotify to display all this, you just need to ask the music distributor whether something needs to be changed in the MP3 TAG

Exactly.
You have to ask the Spotify people what they need. If that is metadata in tags, then MP3tag can help you to add this data.
But all the questions about rules and requirements to publish tracks with the help of a certain distributor has to be asked in their support.
I would have expected that by now you would have followed the link from @Florian 's post:

and that would have answered most of your questions.