How can a PhD holder safely negotiate the salary level for a TVL E13 position? Unlike the question Safely negotiate a salary for a PhD offer - Germany where the goal was to negotiate the % of the salary of a PhD position, my post instead focuses on the specific level (Stufe) of a 100% TVL E13 position offered to a PhD holder.
Additional details:

The field is CS. So, AFAIK first-year PhD students usually start at Stufe 1 of a 100% TVL E13. So, it seems unfair to let a postdoc start at that level too.
My PhD has not been obtained in Germany, but I spent slightly more than three years working on very similar topics/responsibilities as the ones envisioned in the new position.
I have no competing offers with a higher level.

How can I safely negotiate a higher level/Stufe if the human resources put me on the first level? Do I need support from the new professor? Or the PhD advisor (though not in Germany)?
I have read here that this is likely to depend on HR, but if some of you have some tips, they would be more than welcome.
PS: I have also checked this, this and this. But they do not answer the question of how to negotiate, especially if one's PhD is not from Germany.
First, talk to your future supervisor. If they do not support it, chances are slim. Or, if they say it is impossible, equally so.
If they support it: Find out what the rules are - well, probably your supervisor knows.
Generally, there are two options, depending on the place: Either, the administration decides on their own.  Still, they might not proactively put you  in a higher level, but wait for you to ask for it and supply documents proving that you have previous experience.  In that case, the support of your supervisor will certainly help the process. And it is not unlikely that the administration would ask them if they judge the qualification equivalent.
Second, it could be that your supervisor has to write a justification why you should get a higher level, and then the administration decides.  Certainly, in that case the support of your supervisor is essential.
Finally, it could be that the administration has very rigid rules on this, and the supervisor has no influence on that. In particular, they could have the rigid rule that they don't recognize anything which was not in German public service (that would be the most conservative option, since they don't have to vouch for anything).
Good luck!
(In any case, I would say that level 2 is realistic, since, it only requires &gt;1 year of experience. Level 3 is more ambitious, since it requires &gt;3 years of experience.  In any case, also check the salaries for the different levels - the steps are not uniform.)