Find indexes of duplicate strings in C# List

Find indexes of duplicate strings in C# List

I have a List of strings:
["String1"]
["String2"]
["String1"]
["String3"]
["String2"]
["String1"]
That I need to search and find the indexes of "String1" in the List and
also count how many times "String1" occurred. I've reviewed this answer
but I'm new to this type of coding in C# and I'm unclear how to extract
the index values, so if you could explain how to use the solution, that'd
be great!