The idea is to start generating the ones of the first addend from the axiom $S$, while simultaneously adding the corresponding number of ones at the end of the sentential form. Then you have a production from $S$ to non-terminal $A$ which takes care of generating the second added, while still appending ones at the end of the sentential form.
When you are done generating the second addend you can replace $A$ with $=$ in order to split all the ones from the two addends (on the left side) from the corresponding number of ones on the right size.
S &\to 1S1 \mid +A \\
A &\to 1A1 \mid \,=