Update

src/nlp_expr.jl

@@ -1147,9 +1147,16 @@ end
 
 function add_to_expression!(f::GenericNonlinearExpr, args...)
     return error(
-        "`add_to_expression!` is not supported for expressions of type " *
-        "`$(typeof(f))` because they cannot be modified in-place. " *
-        "Instead of `add_to_expression!(expr, args..)`, use " *
-        "`expr += *(args...)`.",
+        """
+        `add_to_expression!` is not supported for expressions of type
+        `$(typeof(f))` because they cannot be modified in-place.
+        Instead of `add_to_expression!(expr, args..)`, use one of the following:
+        ```julia
+        expr += *(args...)
+        # or
+        import MutableArithmetics as MA
+        expr = MA.operate!!(MA.add_mul, expr, args...)
+        ```
+        """,
     )
 end

test/test_nlp_expr.jl

@@ -932,10 +932,17 @@ function test_add_to_expression!()
     y = zero(NonlinearExpr)
     @test_throws(
         ErrorException(
-            "`add_to_expression!` is not supported for expressions of type " *
-            "`$(typeof(y))` because they cannot be modified in-place. " *
-            "Instead of `add_to_expression!(expr, args..)`, use " *
-            "`expr += *(args...)`.",
+            """
+            `add_to_expression!` is not supported for expressions of type
+            `$(typeof(f))` because they cannot be modified in-place.
+            Instead of `add_to_expression!(expr, args..)`, use one of the following:
+            ```julia
+            expr += *(args...)
+            # or
+            import MutableArithmetics as MA
+            expr = MA.operate!!(MA.add_mul, expr, args...)
+            ```
+            """,
         ),
         add_to_expression!(y, 2.0, sin(x)),
     )