Mohamed,

I hope, this link helps:

https://explore.researchgate.net/display/support/Generating+a+DOI

Best

Anna

Thank you Anna-Maria. 

I followed then steps; however, I did not find the right-hand side option of assigning the DOI as stated (Click Generate a DOI for this publication on the right-hand side). Do you know where to find this option please. 

Mohamed,

it is the three dots right hand to "share publication".

Do you find it?

Yes, I found them, but they only allow four options including one for editing, which has a DOI that is blank. Do I need to write up the DOI according to some format?

I mostly use APA. The format might be different in different fields of research and scientific communities.