Here are three methods.

1. This may not be feasible in your case, but you could combine the two forms
into one, as separate pages on a MultiPage control. Then you can refer directly
to the combobox from the other control, because they are members of the same
form.

2. The easiest other way is to use a global variable. In a VBA module in your
workbook, declare a global variable. For example:

Global SelText As String

In the code for the form  with the combobox, use the Change event to set the
value of SelText whenever a selection is made from the combobox. For example:

Private Sub ComboBox1_Change()
SelText$ = Me.ComboBox1.Value
End Sub

In the code for the other form, use the Activate event to update the control
on the form (for example, a textbox) with the current value of SelText. For
example:

Private Sub UserForm_Activate()
Me.TextBox1.Text = SelText$
End Sub

3. Instead of defining a global variable (some programmers object to them),
you could write the combobox value to a worksheet cell (perhaps on a hidden
sheet), then read the value in that cell from the second form.