Not really a command, but there are a few ways to do that.
If you are using kissassist, you can put that in the ini.
If you are using cwtn plugins, you can add the horse as a clicky, but be sure that auto dismount is not checked.
For ihc, i know that some class allow to set that up in the ini, for the other i just click it manually.
If you are using the same horse on all you toons, you can (using eqbc, i don t know for dannet) /bcaa //useitem InsertNameOfYourHorseItemHere