Hi, I am a bit confused by your post.
First, do you mean a starter solenoid or a starter relay? Your motor starter should already have a solenoid on it. A starter relay is meant to protect the ignition switch contacts and can be installed with not much difficulty.
Second, all batteries experience a voltage drop when attempting to start a motor. However, if the voltage drop is severe, it can be attributed to a weak battery, dirty/loose connections, or faulty starter. Just installing a starter relay will not cure the problem if it is due to one of the reasons above.
I would first make sure that the battery is in good condition, charged, and that both + and - terminals are clean and greased, Second, I would ensure that the connection of the ground cable from the battery to the engine block is properly connected and tight on the engine side, and that the terminal is clean and greased. Third, ensure that the cable from the + battery to the starter is tight and the contacts are clean and greased. If all is OK, I would attempt to connect the starter solenoid terminal to the + of the battery. If the starter does not turn or turns with difficulty, it may need new brushes, or get the commutator skimmed on a lathe, or a new starter solenoid.
And of course, let us not overlook a possible faulty battery isolation switch.
Well, this does not cover all possibilities, but I think it is a good start to troubleshoot your problem.
Cheers.
