I would like to give a initial value to a Lookup (multiselect field) dynamically from a user input.

I would like to give a initial value to a Lookup (multiselect field) dynamically from a user input.

In my samples form, I have a look up field sample type, from a sample_type form, which displays the name of all sample type in a single select drop down.
In the same form (samples), there is a look up field parameters, from a parameters form, which displays all parameters, in multiselect list.

But, in the the sample_types form, there is already a look-up parameters field from the parameters form, which displays all parameters, in multiselect list.

A sample type has already the the parameters chosen. But when we create a sample through the samples form, I want to give the option to add few more parameters plus the one already recorded in the sample type.

In the sample form, I see a system like that : When we choose a sample type, the parameters belonged to this type become selected automatically in the multiselect list of the lookup parameters field.
In this way, can know what are the parameters of this sample type, and can add or take off the parameters he wants.

I tried with this tutorial : dynamically adding items to a picklist . But i don't succeed.

Can you help me please?

Thank you.