That doesn’t work that way.
If you order a custom plate for a car with yanktons it will overwrite the yanktons with whichever plate color was selected in iFruit.
The only way to put your text on Yanktons you have not modded is to change the plate text at LSC immediately after receiving it from GCTF.
What you need to do is order the new plate and text on any other of your vehicles with regular plates. Fulfill the order in the game.
Once the text is in the game you can then add that text to the Yanktons.
The key is to not have any other of your friends cars with the plates you received. If you have more than one set of plates of your friends then custom plates will be locked on those particular vehicles.