If there is "some value" in cell F1, it is required to obtain that data as a concatenation of cells B3 and B4, where B3="F" and B4="1".
attempt :
Use the & operator to perform concatenation.
="="&B3&B4
Resulting in "=F1" as a string plus not the data located in said cell.
Short answer
Try INDIRECT
Explanation
INDIRECT converts a string in either A1 or R1C1 notation to a reference.
In this case the formula would be as follows
Note that the operator
=
is only included once, and this is done at the beginning of the formula.