I think OP asked about adding a journal toResearchGate and I'm also interested in this problem (as an author, not as a publisher). The answer given semme completely irrelevant to the question :-(
I have the same problem. I have two publications on Research on Biomedical Engineering (ISSN Online 2446-4740; ISSN Print 2446-4732), published by Springer.
The only way is to mail RG team so that they add this specific journal. (and this way is not possible.. RG provides no support.. and if any, it`s really really bad support).
Try another platform.. I recommend you trying Publons.. I`m not sure if they have this feature, but this one is much better.
All: Once, I found a similar post with this reply/answer:
If your journal isn't visible in the drop-down menu, it's not in our database. We're not currently adding journals to our database by request, but this may change in the future.
@ Janusz S. Bień Publons is a new platform.. Some previous platforms (ORCID, Web of Science etc are merged in this new platform.
I tried it a few days ago, and found that it works differently. You can add any journal by exporting its data (from a BibTeX file etc...) , or by importing it from Google Scholar or other specified (and few) sources.
However, I was able to add the journal by a BibTeX file I created myself (In Publons)
Another platform that works similarly to RG is Academia.. I noticed that many journals note at their websites being indexed in Academia... so, I think that it is much known and much better.. and that journals are frequently and easily added there (by their publishers).
I migrated from Academia to RG for some reasons (DOI, some features there are free here), but, it seems that Academia is better... so, I recommend Academia or Publons.