Nope it’s not a verbe, it’s a noun & the “ha” is to relate the word to the horses.
“Mrabet” is plural for “marbet” = The thread or the wire with which you ride the horse & control it.
According to the contest, it also might refer to the men who ride.
Mrabet also can mean “set/establish & get ready”
Generaly: every word formed with R.B.T has sth to do with “ties” / “connections” / “unit”
The capital Rabat is actually “ribat” & it literally & historically means the place where people were “mrabtin” (gathered, united & getting ready) <— They were known as “moorabitin” & they’re one of the folks who worked on spreading Islam in Morocco after it was brought from the Midle-East.
The movement & its name were inspired from the Qur’anic verse that its contest clearly included the verb “Rabito” in the form of an order.
“ya ayuha 'lladina amanu, isbiro wa sabiro wa “rabito” wa 'tta9o 'llaha la3allakum tofli7oon”
“O those who have believed, be patient & be ready & fear Allah so you can succeed” <— simplified translation.
I hope this was useful :^^: