Why not use the Memo field on your Buy transaction?
That text will show in your investment account register, but only if the register is set to display in 2-line mode. (See Edit / Preferences / Investment transactions)
There also is a Comments field as part of the Security definition itself, but I can't find any report that would display this field anywhere.
Since I buy and download the transaction from my broker I don't see the window you portray. But the comments section of the security definition will do for my reminder purposes.
With the investment register set to two-line mode, the Memo field is in the 2nd transaction line, just below the Security name.Just tab in there and enter your Note then click or hit Enter. (Or click the Edit button on the transaction to bring up the above data entry view.)