SQLyog does not allow setting arguments or parameters for DateTime, TimeStamp, Time data types. The argument is allow the support for high precision microseconds on those types. The Length edit box is uneditable, forcing you to have to do it manually by sql command.
After you alter the table manually from DateTime to DateTime(6), when you use SQLyog's Alter table UI, it will show you the correct DateTime type, with 6 as the value on the Length column, only that it is uneditable.
I'm currently using 11.24 64bit. It is not when adding a new row. It is on the Alter Table screen, under the column Length. If your Data Type is DateTime, Length field is uneditable. If you manually write an Alter Table query and set the DataType to Datetime(6), then 6 will appear on the Length field, which makes sense. So if you want to use the Alter Table or Design Table screen to give DateTime microseconds feature, you will not be able to do it, because the Length field is blocked, you cannot type in it. Don't know if you get what I'm referring too.