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The only way to do that would be to let someone else take care of your account.
Click the little head on top right of the page, click Account and then click Account sharing. There you find the specifics. Only your game friends can be selected.
They do NOT need your login and password for it, NEVER give those to anyone.
You can always withdraw the permission whenever you want.

I can't emphasize this enough: ONLY do this with people you fully trust with your account!!!
They will not be able to access everything while being 'co', but still things can happen that you do not want.

By the way: not much will change while you are gone, your horses for example will just freeze in time and will not die. But yes, boxes, meadows and so on will deteriorate with 1% a day.

Also good to note: Howrse can delete your account when there has been no activity for more than 180 days, so I'd make sure to log in every now and then.
When you are logged in, your co-manager can not.

Just repair your boxes and workshops before you go. As long as you come back within 100 days, it'll be fine.