Hi
Sorry what is WMQ? My brain is too tired to think lol. Also, is the Relationship Record set to Business Required???
I would place a hidden label on the form and put the GUID in there for the Lead Record. This way when the data is submitted, you have it to find the lead record (depending on how you do it).
However, if the Relationship is set to Required, then you actually need to do the Create on the server side, not on the form (is it a patch or what are you doing)
This would mean creating a Plugin that fires before the WMQ record is even created. In this Pre-Post step, you would do your lookup to find the Lead record by using the GUID you had in the form, OR using the email of the submitter (assuming its the same email).
A couple of ways to do it, the plugin is easy, its a small plugin, register the step and done. Loads of examples for this all over but I can help with it if you need.
That or you need to pre-populate the LookUp on the form as it loads, which would be easy enough since you have the email address and (hidden GUID) value in the label.
Then simply use that to create the relationship lookup record (and I am assuming, you have 1 lead with many WMQs or is it the other way?)
Oh actually here is another way. Create the WMQ record and attach it to the lead, as you render the page. That way its just an update to the WMQ row. Just another option