Yes!! You need to fill separate form for NMIMS Mumbai after filling in the NMAT application form.And not only this! You need to apply again separately on NMIMS website if you wish to appear for another window in a season.As NMAT can be appeared 3 times in a year when the exam season starts , so u need to apply every single time on NMAT by Gmac website and on NMIMS website separately or else your score in NMAT exam will not be considered by the institute.All the best