You will need a reference from one of the forms to the other or you will need both forms to share a message deleivery system (e.g. an
event aggregator[
^]
I would recoment the earlier as it is much easier to impliment and you likely are new to programming (no offence).
So to do this the object that is building the forms will need to tell at minimum one of them about the other. Once this is done, the one that holds the reference can update the others data.
so your VB could contain a reference to your C# form and then access an exposed proeprty that gets access to the text box.
The object that is opening or controlling both forms sets up the references. If it is not a single object then you will have to find a way to push the reference down from a point they both share, e.g. where the application starts 'Main'.
VBForm vbForm = new VBForm();
CSharpForm cSharpForm = new CSharpForm();
vbForm.TheCSharpForm = cSharpForm;
...
By sharing the reference the one can access anything you expose. So you can expose the Text Value of said text box publiclly and get access to it from the VBForm property in the C# form.