How to cite a new journal in abbreviation form which is not found in existing abbreviation list?
There is a list of journal abbreviations issues by ISI where official abbreviations are found. There are many places where the list is found but the best updated will likely be the one at Web of Science. your journal is not listed here as far as I can see but abbreviations follow certain guidelines so it would not be difficult to "guess" what abbreviation should be suitable. My guess will be Syst. Sci. Contr. Eng. based on other existing abbreviations including the words that make up the title. 

Note, however, that this is still not the formal abbreviation and I would add a note to the fact to the editor when submitting the manuscript so the journal can decide what to print based on what you have provided. It is possible that the copy-editing will catch this as well but declaring your actions will always help editors in their work and will be appreciated.