It was in the notorious ep 'Spock's Brain'. An absolutely beautiful actress named Sheila Leighton was playing Luma, the first girl the Enterprise crew spot on the frigid planet. Just after they first see her there is a close up of Leighton's face and she does indeed look like the future Wonder Woman but with browner colored hair. However, Leighton is too diminutive, different voiced and age-wise is all wrong to be beauty star Lynda Carter.

Well, they're both female...

The similarity between the beautiful actresses was was much stronger than just that, Torgo.

Assuming everyone knows what Lynda Carter looks like, may as well judge for yourself.....

https://i.ebayimg.com/images/g/npUAAMXQUY1Q9~7O/s-l300.jpg

More like TV movie Wonder Woman, Cathy Lee Crosby...

Leighton looks nothing at all like Cathy Lee Crosby. But the former's photo shows what a lookalike she is to Ms. Carter.