The login credentials used for logging into IGAM are the same as FIMS.
To give yourself or another user access to IGAM, you do that within FIMS:
Tools > System Utilities > Establish System Users
click the edit user button after selecting the appropriate user.
Under the IGAM Group setting, select the level access (ex Full Permissions) and what rights to IGAM conditions: (ex normal ones are Publish/Refresh/Create/Delete).
Then Save and close.