Supposedly, you could use any vocalisation (a/u/i) between the m and the h. I found this video explaining at 3:26 minute. https://www.youtube.com/watch?v=GCsOTt1oSBQ&list=PL0A4EB5D68AF2E67E&index=17&t=0s
In this context, it should be "2ismuhaa" because it is in the nominative case (ie, as the subject or 2al-mubtada2) -- if we follow Standard.
("2ismuhaa Tawiil(un)" means: "Her name is long".)
Whilst, if it is accusative (e.g., the object or maf3ul bihi), it wil be "2ismahaa":
("2a3rifusmahaa" means: "I know her name".)