According to Shakespeare's Romeo and Juliet their love was both fate and free will, but can anyone give me evidence of how it was free will please?

It is very difficult to argue that Romeo and Juliet's love was purely based on free will because the Prologue clearly states that their doom is written in the stars. But that doesn't hold the lovers back from attempting to exercise their free will. The phrase "free will" suggests that a person can make a conscious choice on his or her own without the influence of fate or the universe. There are  a couple of times in the play that the lovers seem to make comments or choices based on their own conscious desires in order to fight against fate.

One specific time that Juliet tries to take her destiny into her own hands is when she threatens to kill herself in front of Friar Lawrence. At this point Capulet is forcing Juliet to marry Paris soon, yet she is already married to a banished Romeo. Rather than give herself over to Fate, Juliet desires to end it all with a knife she is holding. She says the following:

"God join'd my heart and Romeo's, thou our hands;

And ere this hand, by thee to Romeo's seal'd,

Shall be the label to another deed,

Or my true heart with treacherous revolt

Turn to another, this shall slay them both" (IV.i.57-61).

When Juliet says "treacherous revolt," she's saying that her "true heart" will betray Fate's decided path for her. Up until this point she has believed that Fortune and Fate have had a hand in bringing Romeo into her life. But she can't take being blindly faithful, now and thinks that she can take control of her Fate by making her own decision.

Romeo has a similar experience when he is pushed too far with frustration. for example, when Balthasar tells Romeo that Juliet is dead, he can't take what life throws at him any longer. He says the following:

"Is it even so? Then I defy you, stars!

Thou know'st my lodging; get me ink and paper

And hire post-horses; I will hence to-night" (V.i.24-26).

Thus, Romeo recognizes that Fate has had a hand in his life, but he believes that he can make choices against it. Just like Juliet, he chooses to end his life rather than live without her. One could argue that this is where he uses free will; but again, it's really difficult because it can be argued the other way as well--that he's just playing into Fate's prophecy from the Prologue.