Thereare two ways to reference an ActivX textbox:

The textbox will be on a slide object whose name you will be able to see selected in the vba editor (project explorer pane) if you right click the textbox > view code. It will look like Slide1 but note Slide1 does not necessarily = the first slide.

You can then use:
[VBA]Slide1.TextBox1.Text="abc"[/VBA]

If you want to refer directly to a TextBox1 on the first slide in the presentation use:

[VBA]ActivePresentation.Slides(1).Shapes("TextBox1").OLEFormat.Object.Text = "abc"[/VBA]