How to assign designers name in a Solidworks part file(.sldprt)?

As SolidTweaks said, all of the options I can think of to tag a file with your name are at best temporary.
It really kind of depends on how smart your adversary is. This is a situation where your only security is coming from obscurity. Once they know where/how you tag the files, they can modify the data.

File properties, custom properties, design journal, hidden annotations, sketches, features... All of these are locations where your name could be hidden.

The "most secure" option might be to rely simply on Feature Properties. Right click any feature in the Feature Tree, and choose Properties. It will give a time/date stamp as well as the name of the creator. In this example, my user name is default.

It is not permanent, but you can use the Custom Properties of the file.

What is the goal of adding in your name? Maybe with some additional details we'll have a better options.

You can add a custom property or use the 'drawn by' or 'author' property as a place to include your name but then you're probably forced to send the native solidworks part to the other company.

Sometimes this is preferred, but other times, such as when dealing with suppliers, it's safer to send a static output such as step or iges, which loses the cad file custom properties.

Perhaps there's some other metadata that can be added to the file after-the-fact, I see a spot for 'authors' in windows file properties (right click, properties, details tab), but I can't seem to adjust that data here at work, perhaps our IT doesn't allow it.

Here is an image of what Robert suggested.
I was able to add text to each of the entries.

Again, both options are pretty temporary. Anyone one could just delete the entries in the Windows properties, or the SolidWorks Custom Properties.

I was thinking this was being done as proof of who worked on, or owned the model

Each feature gets automatically tagged with 'created by, 'date created' and 'last modified' metadata, which doesn't seem to be readily modifiable.

Right click any feature in the tree and select 'feature properties' to review the metadata.

Maybe that is enough for your situation?

I am afraid that there are no "permanent" solution to your question. Even if you get to have your details in the properties, there are many software that enable one to change the attributes easily. Maybe if you can explain the premises of your application, we can see, at best, up to what level you can ensure that it is your drawing.

i have changed the created by name but for files created using solidworks 2013 but in 2016 it is really harder as the hex strings are changed in newer versions unfortunately