Creating a search box that filters a Sharepoint lookup field

I'm fairly new to Power Apps, and I'm trying to create a search box that will search a gallery. I'm using the following formula for my gallery's item property:

SortByColumns(Filter([@'Physical Plant Reviews'], StartsWith(Site, TextSearchBox1.Text)), "Site", If(SortDescending1, Descending, Ascending))

Physical Plant Reviews is my Sharepoint list, and the field I'm trying to search is called Site. I suspect the problem is that "Site" is a lookup field. When I use the same formula but replace Site with another regular text column, the search function works fine.

Is it possible to create a search box that will filter results from a lookup field in Sharepoint?

Thank you!