Linking text in a document

Hi there

I am wanting to link part of a text form field in a template to another text form field at the end of my template as follows:

Dear Sirs needs to be linked to the close of the letter: Yours faithfully

However, if the person writing the letter knows the persons name, I need to leave them the option of typing in the form field the following:

Dear Name - this needs to be linked to the close of the letter: Yours sincerely.

How can I go about this? I almost need to insert an IF function as you use in Excel. I have tried the IF field but I dont think this does exactly the same as the excel IF function.

Please advise.

Many thanks