In the DataFrames.jl documentation are quite a few examples on how to join two data frames.
https://dataframes.juliadata.org/stable/man/joins/
If I read your example right you want to innerjoin the two data frames A and B.
julia> A = DataFrame(Timestamp = [1, 2, 3], Ticker = ["a", "b", "c"], Price = [1.0, 2.0, 3.0])
3×3 DataFrame
Row │ Timestamp Ticker Price
│ Int64 String Float64
─────┼────────────────────────────
1 │ 1 a 1.0
2 │ 2 b 2.0
3 │ 3 c 3.0
julia> B = DataFrame(Timestamp = [2, 3, 4], Ticker = ["b", "c", "a"], ProfitMargin = [10.0, 20.0, 30.0])
3×3 DataFrame
Row │ Timestamp Ticker ProfitMargin
│ Int64 String Float64
─────┼─────────────────────────────────
1 │ 2 b 10.0
2 │ 3 c 20.0
3 │ 4 a 30.0
julia> C = innerjoin(A, B, on = [:Timestamp, :Ticker])
2×4 DataFrame
Row │ Timestamp Ticker Price ProfitMargin
│ Int64 String Float64 Float64
─────┼──────────────────────────────────────────
1 │ 2 b 2.0 10.0
2 │ 3 c 3.0 20.0