Unfortunately, the standard steemit UI only lets you do this if you have some minimum amount of steem power in your account :(. Not sure what figure isrequired for this .... maybe 500SP, 1000SP, or something else - you'd need to investigate. Also gives you a slider so you can vote with less than 100%. Here's a post about it:
https://steemit.com/steemit/@razvanelulmarin/we-have-a-slider-what-does-it-do